Display device and electronic device

ABSTRACT

A display device that has an excellent visibility even under strong light is provided. In the display device, a first display element that reflects visible light and a second display element that emits visible light are between a first substrate and a second substrate. The display device can display an image with high visibility by operating the first display element under strong light and operating the second display element under weak light. Furthermore, a first surface of the second substrate is provided with a touch sensor, and a second surface opposite to the first surface is provided with an anti-reflection layer. Such a structure can sufficiently reduce reflection of external light on the display surface under strong light, further increasing the visibility.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to an object, a method, or a manufacturingmethod. The present invention relates to a process, a machine,manufacture, or a composition of matter. In particular, one embodimentof the present invention relates to a semiconductor device, alight-emitting device, a display device, an electronic device, alighting device, a driving method thereof, or a manufacturing methodthereof. In particular, one embodiment of the present invention relatesto a display device (display panel) capable of displaying images on acurved surface. Another embodiment of the present invention relates toan electronic device, a light-emitting device, or a lighting device thatincludes a display device capable of displaying images on a curvedsurface, or a manufacturing method thereof.

In this specification and the like, a semiconductor device generallymeans a device that can function by utilizing semiconductorcharacteristics. A transistor, a semiconductor circuit, an arithmeticdevice, a memory device, and the like are each an embodiment of thesemiconductor device. A light-emitting device, a display device, anelectronic device, a lighting device, and an electronic device mayinclude a semiconductor device.

2. Description of the Related Art

Recent widespread use of electronic devices such as smartphones andtablet terminals gives more and more opportunities for outdoor datacommunication. Furthermore, in the field of display devices included inelectronic devices, techniques for reducing power consumption have beencompetitively developed to achieve long-time operation with a limitedbattery capacity. For example, Patent Document 1 discloses a low-powerliquid crystal display device in which a transistor including an oxidesemiconductor and having a low off-state current is used for a pixel sothat an image signal is retained for a long time.

REFERENCE Patent Document

[Patent Document 1] Japanese Published Patent Application No.2011-141522

SUMMARY OF THE INVENTION

Transmissive liquid crystal elements with backlights as light sources orself-luminous organic electroluminescent (EL) elements are widely usedin display devices of electronic devices. Such display elements offer anexcellent visibility indoors; however, the visibility of light (display)exhibited from the inside of the display device decreases under stronglight, e.g., outdoors in fine weather, because reflection of externallight on the display surface increases in intensity under such stronglight.

Thus, under strong light, it is preferable to use a reflective displayelement that utilizes reflection of external light. For example, adisplay device with a reflective liquid crystal element has a highervisibility when used in external light with a higher intensity. However,the display surface of the display device includes a glass substrate, aresin substrate, or the like that has a reflectance of several percent,and thus the influence of external light reflection on display stillremains.

In addition, a reflective display element does not have an adequatevisibility under low-intensity external light. Therefore, a transmissiveliquid crystal element or a self-luminous organic EL element ispreferably used in combination with the reflective display element sothat an appropriate display element can be selected in accordance with achange in environment to display an image.

An object of one embodiment of the present invention is to provide adisplay device that has an excellent visibility even under strong light.Another object is to provide a display device that includes a displayelement having a function of emitting visible light and a displayelement having a function of reflecting visible light. Another object isto provide a low-power display device. Another object is to provide anovel display device. Another object is to provide a novel electronicdevice.

Note that the descriptions of these objects do not disturb the existenceof other objects. In one embodiment of the present invention, there isno need to achieve all the objects. Objects other than the above objectswill be apparent from and can be derived from the description of thespecification and the like.

One embodiment of the present invention relates to a display devicehaving a function of emitting visible light, a display device having afunction of reflecting visible light, and a display device having afunction of emitting visible light and a function of reflecting visiblelight. Furthermore, an embodiment of the present invention relates to anelectronic device including the display device.

One embodiment of the present invention is a display device including afirst substrate, a second substrate, a first display element, a seconddisplay element, an input device, and a driver circuit. The firstsubstrate and the second substrate overlap with each other. The firstdisplay element and the second display element are between a firstsurface of the first substrate and a first surface of the secondsubstrate. The first display element is configured to reflect visiblelight, and the second display element is configured to emit visiblelight. The input device is between the first surface of the secondsubstrate and the first and second display elements. A second surface ofthe second substrate opposite to the first surface of the secondsubstrate is provided with an anti-reflection layer. The first surfaceof the first substrate is provided with the driver circuit. The inputdevice and the driver circuit are electrically connected to each otherthrough an FPC.

The first display element and the second display element can be in thesame pixel unit.

The driver circuit can be configured to drive the first display element,the second display element, and the input device.

The first surface of the second substrate may be provided with ananti-reflection layer.

The anti-reflection layer can be formed with a dielectric layer.Alternatively, the anti-reflection layer may have an anti-glare pattern.

The input device includes a wiring including a first layer provided forthe first surface of the second substrate and a second layer in contactwith the first layer. The first layer preferably contains a materialwhose reflectance is lower than that of the second layer.

A light diffusion plate and a polarizing plate are preferably betweenthe input device and the first and second display elements.

Each of the first display element and the second display element ispreferably electrically connected to a transistor whose semiconductorlayer including a channel contains a metal oxide.

Note that in this specification, the display device includes any of thefollowing modules in its category: a module in which a connector such asa flexible printed circuit (FPC) or a tape carrier package (TCP) isattached to a substrate over which a display element (display portion)is formed; a module having a TCP provided with a printed wiring board atthe end thereof; and a module having an integrated circuit (IC) directlymounted by a chip on glass (COG) method on a substrate over which adisplay element is formed.

According to one embodiment of the present invention, a display devicethat has a high visibility even under strong light can be provided. Adisplay device that includes a display element capable of emittingvisible light and a display element capable of reflecting visible lightcan be provided. A display device with low power consumption can beprovided. A novel display device can be provided. A novel electronicdevice can be provided.

Note that the description of these effects does not preclude theexistence of other effects. One embodiment of the present invention doesnot necessarily achieve all the effects listed above. Other effects willbe apparent from and can be derived from the description of thespecification, the drawings, the claims, and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B illustrate a display device.

FIGS. 2A to 2F each illustrate an anti-reflection layer.

FIG. 3 illustrates a display device.

FIGS. 4A to 4D each illustrate an example of connection between a drivercircuit and an FPC.

FIGS. 5A to 5C illustrate idling stop driving.

FIG. 6 illustrates pixel units.

FIGS. 7A to 7C illustrate a pixel unit.

FIG. 8A illustrates a circuit of a display device and FIGS. 8B1 and 8B2are top views of pixels.

FIG. 9 illustrates a circuit of a display device.

FIG. 10A illustrates a circuit of a display device, and FIG. 10B is atop view of a pixel.

FIG. 11 illustrates a structure of a display device.

FIG. 12 illustrates a structure of a touch sensor.

FIGS. 13A and 13B illustrate a structure of a touch sensor.

FIG. 14 illustrates a structure of a display device.

FIG. 15 is a conceptual diagram of a composition of a metal oxide.

FIG. 16 shows measured XRD spectra of samples.

FIGS. 17A and 17B are TEM images of samples and FIGS. 17C to 17L areelectron diffraction patterns of the samples.

FIGS. 18A to 18C show EDX mapping images of a sample.

FIGS. 19A1, 19A2, 19B1, 19B2, 19C1, and 19C2 each illustrate atransistor.

FIGS. 20A1, 20A2, 20B1, 20B2, 20C1, and 20C2 each illustrate atransistor.

FIGS. 21A1, 21A2, 21A3, 21B1, and 21B2 each illustrate a transistor.

FIGS. 22A1, 22A2, 22B1, and 22B2 each illustrate a transistor.

FIGS. 23A1, 23A2, 23A3, 23B1, 23B2, 23C1, and 23C2 each illustrate atransistor.

FIGS. 24A1, 24A2, 24B1, 24B2, 24C1, and 24C2 each illustrate atransistor.

FIGS. 25A1 and 25A2 illustrate a transistor.

FIGS. 26A and 26B each illustrate a structure of a display module.

FIGS. 27A to 27F each illustrate an electronic device.

FIGS. 28A and 28B each illustrate an electronic device.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments will be described in detail with reference to the drawings.Note that the present invention is not limited to the followingdescription and it will be readily appreciated by those skilled in theart that modes and details can be modified in various ways withoutdeparting from the spirit and the scope of the present invention.Therefore, the present invention should not be interpreted as beinglimited to the description of embodiments below.

Note that in the structures of the invention described below, the sameportions or portions having similar functions are denoted by the samereference numerals in different drawings, and description of suchportions is not repeated. Further, the same hatching pattern is appliedto portions having similar functions, and the portions are notespecially denoted by reference numerals in some cases.

Note that in each drawing described in this specification, the size, thelayer thickness, or the region of each component is exaggerated forclarity in some cases. Therefore, the size, the layer thickness, or theregion is not limited to the illustrated scale.

Note that in this specification and the like, ordinal numbers such as“first,” “second,” and the like are used in order to avoid confusionamong components and do not limit the number.

Embodiment 1

In this embodiment, a display device according to one embodiment of thepresent invention will be described with reference to drawings.

A display device of one embodiment of the present invention includes afirst substrate, a second substrate, a first display element, a seconddisplay element, a touch sensor, and a driver circuit.

The first display element has a function of reflecting visible light.The second display element has a function of emitting visible light.Thus, the display device can display a highly visible image with lowpower consumption by, for example, operating the first display elementunder strong light and operating the second display element under weaklight.

The first display element, the second display element, and the touchsensor are between the first substrate and the second substrate. A firstsurface of the second substrate is provided with the touch sensor, and asecond surface opposite to the first surface is provided with ananti-reflection layer. Such a structure can sufficiently reducereflection of external light on the display surface under strong light,further increasing the visibility.

FIG. 1A illustrates a display device of one embodiment of the presentinvention. A display device 10 in FIG. 1A includes a first substrate 11,a second substrate 12, a layer 20, a driver circuit 30, an FPC 31, andan FPC 32.

The first substrate 11 and the second substrate 12 are glass substrates,for example. Alternatively, they may be flexible resin substrates. Notethat at least the second substrate 12 contains a light-transmittingmaterial, because the second substrate 12 side of the display device 10is a display side (viewing side).

A first surface and a second surface of the second substrate 12 eachare, or the second surface is, provided with an anti-reflection layer13. The anti-reflection layer 13 may have a structure of, for example,any of layers 13 a to 13 d shown in FIGS. 2A to 2F.

FIG. 2A shows an example where the second surface of the secondsubstrate 12, which is a top surface of the display device 10, isprovided with a dielectric layer 13 a having a light-transmittingproperty. The dielectric layer 13 a is a multi-layer dielectric layerwith an appropriate thickness; accordingly, light reflection can bereduced by a light interference effect. The reflectance of a surface ofthe glass substrate is approximately 4% to 5%; the reflectance can bereduced to approximately 0.05% to 0.5% by providing thelight-transmitting dielectric layer 13 a for the second surface of thesecond substrate 12.

When the first surface of the second substrate 12 is also provided witha dielectric layer 13 b having a light-transmitting property as shown inFIG. 2B, the reflectance of the rear surface of the glass substrate canbe reduced. In that case, the reflectance of the front and rear surfacesof the second substrate 12 can be reduced to approximately 0.1% to 1.0%.Therefore, glare of external light can be reduced and display visibilitycan be improved.

Alternatively, as shown in FIG. 2C, the second surface of the secondsubstrate 12 may have an anti-glare pattern 13 c with minuteprojections. The anti-glare pattern 13 c can scatter reflected light;thus, the visibility of display by a reflective display element canincrease. In addition, the anti-glare pattern 13 c can reduce attachmentof smudges such as fingerprints. Although the example of FIG. 2C showsthat the anti-glare pattern 13 c is formed by processing the secondsurface of the second substrate 12, a film 13 d having an anti-glarepattern may be attached to the second surface of the second substrate 12as shown in FIG. 2D.

Alternatively, the anti-glare pattern 13 c and the dielectric layer 13 bmay be used in combination as shown in FIG. 2E. Alternatively, the film13 d having an anti-glare pattern and the dielectric layer 13 b may beused in combination as shown in FIG. 2F.

The layer 20 is between the first substrate 11 and the second substrate12. The layer 20 is described with reference to FIG. 1B. FIG. 1Bcorresponds to a cross section along line X1-X2 in FIG. 1A. The layer 20includes an element layer 21, a substrate 22, a light diffusion plate23, a polarizing plate 24, a touch sensor 25, and an adhesive layer 26.

The element layer 21 includes an FET layer 21 a, an LC layer 21 b, andan OLED layer 21 c. The FET layer 21 a includes, for example, atransistor constituting a pixel circuit. The LC layer 21 b includes thefirst display element. The OLED layer 21 c includes the second displayelement. The first and second display elements are electricallyconnected to transistors in the FET layer 21 a.

The first display element is, for example, a reflective liquid crystalelement. The second display element is, for example, a light-emittingelement. The reflective liquid crystal element consumes a low amount ofpower, and can perform display with high visibility even under sunlightin fine weather. The light-emitting element can perform display withhigh visibility under indoor light, outdoor light in cloudy weather, orthe like.

The substrate 22 has a function of sealing a liquid crystal layerincluded in the first display element. The substrate 22 can be a glasssubstrate or a resin substrate such as a film, for example.

The light diffusion plate 23 has a function of diffusing light reflectedon a reflective electrode of the liquid crystal element. Such a functionallows the reflective liquid crystal element to exhibit a natural colorand display a white color close to that of white paper.

The polarizing plate 24 is, for example, a circularly polarizing plate.Change in deflection angle owing to the circularly polarizing plate andthe liquid crystal is utilized to enable display with reflected light.

The touch sensor 25 is, for example, a capacitive touch sensor. Thetouch sensor 25 is an input device, and overlaps with a display portion.The touch sensor 25 has a function of converting a touch to the displayportion by a user into an electric signal and outputting the signal.

The touch sensor 25 is provided for the first surface of the secondsubstrate 12 as shown in FIG. 3. Alternatively, the touch sensor 25 maybe provided for the above-mentioned dielectric layer 13 b. In the caseof a capacitive touch sensor, its wiring and electrode may employ alight-transmitting conductive film, but preferably employ a metal meshthat has a lower resistance than the light-transmitting conductive filmand is available for a large-size display device. A metal, typicallyhaving a high reflectance, can have a dark color through oxidationtreatment or the like. Thus, even when the touch sensor 25 is providedfor the first surface of the second substrate 12, reflection of externallight can be reduced.

The touch sensor 25 is an external touch sensor, and overlaps with thefirst and second display elements with the visible light-transmittingadhesive layer 26 between the touch sensor 25 and the first and seconddisplay elements. The touch sensor 25 can be manufactured separatelyfrom the manufacturing process for the first and second displayelements; therefore, the yield of each component can be improved.

The driver circuit 30 has a function as a source driver that suppliesimage data to the first and second display elements, and may have afunction of controlling the touch sensor 25. An IC chip formed with asilicon wafer can be mounted as the driver circuit 30. Alternatively,the driver circuit 30 may be formed with transistors over the firstsubstrate 11.

Although FIGS. 1A and 1B and FIG. 3 show an embodiment where a bare chipas the driver circuit 30 is mounted by the COG method, the bare chip maybe provided by the TCP method or the COF method.

The driver circuit 30 is electrically connected through the FPC 31 to,for example, a circuit that supplies image data. The touch sensor 25 iselectrically connected through the FPC 32 to the driver circuit 30.

FIGS. 4A to 4D each illustrate electrical connection among the drivercircuit 30, the FPC 31, and the FPC 32.

FIG. 4A shows an example where the driver circuit 30 has a function as asource driver that supplies image data to the first and second displayelements and a function of controlling the touch sensor 25. Here, thedriver circuit 30 is electrically connected to the FPC 31 through awiring 33 a. In addition, the driver circuit 30 is electricallyconnected to the FPC 32 through a wiring 33 b.

FIG. 4B shows an example where the driver circuit 30 is divided into twoparts. Here, a driver circuit 30 a has a function as a source driverthat supplies image data to the first and second display elements. Adriver circuit 30 b has a function of controlling the touch sensor 25.The driver circuit 30 a is electrically connected to the FPC 31 throughthe wiring 33 a. The driver circuit 30 b is electrically connected tothe FPC 32 through the wiring 33 b.

As shown in FIG. 4C, the driver circuits 30 a and 30 b may beelectrically connected to each other through a wiring 33 c. As shown inFIG. 4D, the FPCs 31 and 32 may be electrically connected to each otherthrough a wiring 33 d. Such a structure enables a reduction in thenumber of wirings for supplying a power source voltage or a signal.

A transistor in the FET layer 21 a is preferably a transistor whosechannel region includes a metal oxide (such a transistor is hereinafterreferred to as an OS transistor). An OS transistor has an extremely lowoff-state current, with which a potential written as image data can beretained for a long time. The use of an OS transistor enables what iscalled idling stop driving by which image display can be maintained fora plurality of frame periods without rewriting of image data.

The idling stop driving enables retention of image data written in apixel for two or more frames. This reduces the frequency of image datarewriting and thus lowers power consumption.

The reflective liquid crystal element that can be used as the firstdisplay element does not need a backlight, and accordingly the powerconsumption in the pixel portion is equivalent to the power consumptionby circuit operations. Thus, a pixel with the first display element isparticularly preferable to be subjected to the idling stop driving. Inthat case, the power consumption in the pixel portion can decrease inproportion to the rewriting frequency.

An example of the above-mentioned idling stop driving will be describedwith reference to FIGS. 5A to 5C.

FIG. 5A is a circuit diagram of a pixel including a liquid crystalelement 35 and a pixel circuit 36. FIG. 5A illustrates a transistor M1connected to a signal line SL and a gate line GL, a capacitor Cs_(LC),and a liquid crystal element LC.

FIG. 5B is a timing chart showing waveforms of signals supplied to thesignal line SL and the gate line GL in a normal driving mode that doesnot perform the idling stop driving. In the normal driving mode, anormal frame frequency (e.g., 60 Hz) can be used for operation.

The frame frequency includes successive frame periods T₁, T₂, and T₃. Ineach of the frame periods, a scan signal is supplied to the gate lineand data D₁ of the signal line is written to the pixel. This operationis performed both to write the same data D₁ in the periods T₁ to T₃ andto write different data in the periods T₁ to T₃.

FIG. 5C is a timing chart showing waveforms of signals supplied to thesignal line SL and the gate line GL in the idling stop driving. In theidling stop driving, a low frame frequency (e.g., 1 Hz) can be used foroperation.

FIG. 5C shows a frame period T₁ in the frame frequency, which includes aperiod T_(W) for writing data and a period T_(RET) for retaining data.The idling stop driving is performed as follows: in the period T_(W), ascan signal is supplied to the gate line and the data D₁ of the signalline is written to the pixel, and in the period T_(RET), the gate lineis fixed to a low-level voltage so that the transistor M1 is off and thewritten data D₁ is retained in the pixel.

The use of an OS transistor as the transistor M1 enables retention ofthe data D₁ for a long time, owing to its low off-state current.Although FIGS. 5A to 5C show the example where the liquid crystalelement LC is used, the idling stop driving is available even when alight-emitting element such as an organic EL element is used.

In the circuit diagram illustrated in FIG. 5A, the liquid crystalelement LC might serve as a leakage path of data D₁. Therefore, toperform the idling stop driving appropriately, the resistivity of theliquid crystal element LC is preferably higher than or equal to1.0×10¹⁴Ω·cm.

As the metal oxide used for the above-mentioned transistor, acloud-aligned composite oxide semiconductor (CAC-OS) described later canbe used, for example.

In particular, an oxide semiconductor having a wider band gap thansilicon is preferably used. When a semiconductor material having a widerband gap and a lower carrier density than silicon is used, the off-statecurrent of the transistor can be reduced.

Charge accumulated in a capacitor through the transistor can be retainedfor a long time because of the low off-state current of the transistor.The use of such a transistor in pixels allows a driver circuit to stopwhile the gray level of an image displayed in display regions ismaintained. As a result, an electronic device with extremely low powerconsumption can be obtained.

A semiconductor device such as a transistor used in the above-mentionedpixel or circuit for driving the pixel may include a polycrystallinesemiconductor. For example, polycrystalline silicon or the like ispreferably used. Polycrystalline silicon can be formed at a lowertemperature than single crystal silicon and has higher field effectmobility and higher reliability than amorphous silicon. When such apolycrystalline semiconductor is used for a pixel, the aperture ratio ofthe pixel can be improved. Even when a very large number of pixels areprovided, a gate driver circuit and a source driver circuit can beformed over a substrate where the pixels are formed, so that the numberof components of an electronic device can be reduced.

The display device with the above-mentioned structure can display animage with high visibility regardless of the intensity of externallight. The display device can have high visibility under strong lightand operate with low power consumption, which is particularlyadvantageous.

At least part of this embodiment can be implemented in combination withany of the other embodiments and the other examples described in thisspecification as appropriate.

Embodiment 2

This embodiment will describe a display device and a driving method ofthe display device of one embodiment of the present invention.

The display device of one embodiment of the present invention caninclude a pixel in which the first display element that reflects visiblelight is provided. Alternatively, the display device can include a pixelin which the second display element that emits visible light isprovided. Alternatively, the display device can include a pixel in whichthe first and second display elements are provided.

In this embodiment, a display device including the first display elementthat reflects visible light and the second display element that emitsvisible light is described.

The display device has a function of displaying an image with the use ofone or both of first light reflected by the first display element andsecond light emitted by the second display element. Alternatively, thedisplay device has a function of expressing gray scales by individuallycontrolling the amount of first light reflected by the first displayelement and the amount of second light emitted by the second displayelement.

It is preferable that the display device have a structure including afirst pixel expressing gray scales by controlling the amount of lightreflected by the first display element and a second pixel expressinggray scales by controlling the amount of light emitted by the seconddisplay element. The first pixels are arranged in a matrix and thesecond pixels are arranged in a matrix to form a display portion, forexample.

The number of the first pixels is preferably the same as that of thesecond pixels, and the first pixels and the second pixels are preferablyarranged in a display region with the same pitch. Here, the first pixeland the second pixel adjacent to each other can be collectively referredto as a pixel unit. Thus, as described later, an image displayed only bya plurality of first pixels, an image displayed only by a plurality ofsecond pixels, and an image displayed by both the plurality of firstpixels and the plurality of second pixels can be displayed in the samedisplay region.

As the first display element included in the first pixel, an elementthat performs display by reflecting external light can be used. Such anelement does not include a light source and thus power consumption indisplay can be significantly reduced.

As the first display element, a reflective liquid crystal element can betypically used. Alternatively, the first display element can be, forexample, a Micro Electro Mechanical Systems (MEMS) shutter element, anoptical interference MEMS element, or an element using a microcapsulemethod, an electrophoretic method, an electrowetting method, anElectronic Liquid Powder (registered trademark) method, or the like.

As the second display element included in the second pixel, an elementthat performs display with the use of light from a light source in theelement can be used. Specifically, it is preferable to use anelectroluminescence element where light can be extracted from alight-emitting substance by application of an electric field. Since theluminance and the chromaticity of light emitted from such a pixel arenot affected by external light, an image with high color reproducibility(a wide color gamut) and a high contrast, i.e., a clear image, can bedisplayed.

The second display element can be a self-luminous light-emitting elementsuch as an organic light-emitting diode (OLED), a light-emitting diode(LED), a quantum-dot light-emitting diode (QLED), or a semiconductorlaser. Alternatively, the display element in the second pixel may beformed by a combination of a backlight that is a light source and atransmissive liquid crystal element that controls the amount of lighttransmitted from the backlight.

The first pixel can include, for example, a subpixel exhibiting light ofwhite (W), or subpixels exhibiting light of three colors of red (R),green (G), and blue (B). Similarly, the second pixel can include, forexample, a subpixel exhibiting light of white (W), or subpixelsexhibiting light of three colors of red (R), green (G), and blue (B).Note that the first pixel and the second pixel may each includesubpixels of four colors or more. The increased number of subpixelsleads to a reduction in power consumption and improvement in colorreproducibility.

One embodiment of the present invention can switch between a first modein which an image is displayed by the first pixels, a second mode inwhich an image is displayed by the second pixels, and a third mode inwhich an image is displayed by the first pixels and the second pixels.In addition, different image signals can be input to the first andsecond pixels to display a composite image.

The first mode is a mode in which an image is displayed utilizing lightreflected from the first display element. The first mode does notrequire a light source and thus is a driving mode with extremely lowpower consumption. The first mode is effective in the case where, forexample, external light has a sufficiently high illuminance and is whitelight or light near white light. The first mode is a display modeappropriate for displaying text data of a book or a document, forexample. The use of reflected light enables eye-friendly display,thereby mitigating eye fatigue.

The second mode is a mode in which an image is displayed utilizing lightemitted from the second display element. Thus, an extremely clear image(with high contrast and high color reproducibility) can be displayedregardless of the illuminance and chromaticity of external light. Forexample, the second mode is effective in the case where the illuminanceof external light is extremely low, e.g., during the night or in a darkroom. When a bright image is displayed under weak external light, a usermay feel that the image is too bright. To prevent this, an image ispreferably displayed with a reduced luminance in the second mode. Thisprevents too bright display and reduces power consumption. The secondmode is a mode suitable for displaying a clear image and a smooth movingimage.

The third mode is a mode in which an image is displayed utilizing bothlight reflected from the first display element and light emitted fromthe second display element. Specifically, the display device is drivenin such a way that light casted by the first pixel and light emitted bythe second pixel adjacent to the first pixel are mixed to express onecolor. The third mode can offer clearer image display than the firstmode and consume lower power than the second mode. For example, thethird mode is effective when the illuminance of external light isrelatively low, e.g., under indoor illumination or in the morning orevening, or when the external light does not represent a whitechromaticity.

A more specific example of one embodiment of the present invention willbe described below with reference to drawings.

Structure Example of Display Device

FIG. 6 illustrates a pixel array 40 included in the display device ofone embodiment of the present invention. The pixel array 40 includes aplurality of pixel units 45 arranged in a matrix. The pixel unit 45includes a pixel 46 and a pixel 47.

FIG. 6 shows an example where the pixel 46 and the pixel 47 each includedisplay elements corresponding to three colors of red (R), green (G),and blue (B).

The pixel 46 includes a display element 46R corresponding to red (R), adisplay element 46G corresponding to green (G), and a display element46B corresponding to blue (B). The display elements 46R, 46G, and 46Bare each the second display element that utilizes light from a lightsource.

The pixel 47 includes a display element 47R corresponding to red (R), adisplay element 47G corresponding to green (G), and a display element47B corresponding to blue (B). The display elements 47R, 47G, and 47Bare each the first display element that utilizes reflection of externallight.

That is the description of the structure example of the display device.

Structure Example of Pixel Unit

Next, the pixel unit 45 is explained with reference to FIGS. 7A to 7C.FIGS. 7A to 7C are schematic views illustrating a structure example ofthe pixel unit 45.

The pixel 46 includes the display elements 46R, 46G, and 46B. Thedisplay element 46R includes a light source and emits red light R2 tothe display surface side; the red light R2 has a luminance according toa gray level corresponding to a red color included in a second graylevel input to the pixel 46. Similarly, the display element 46G and thedisplay element 46B emit green light G2 and blue light B2, respectively,to the display surface side.

The pixel 47 includes the display elements 47R, 47G, and 47B. Thedisplay element 47R reflects external light and casts red light R1 tothe display surface side; the red light R1 has a luminance according toa gray level corresponding to a red color included in a first gray levelinput to the pixel 47. Similarly, the display element 47G and thedisplay element 47B cast green light G1 and blue light B1, respectively,to the display surface side.

[First Mode]

FIG. 7A illustrates an example of an operation mode in which the displayelements 47R, 47G, and 47B that reflect external light are driven todisplay an image. In the case where, for example, the illuminance ofexternal light is sufficiently high, the pixel unit 45 does not drivethe pixel 46 and mixes only light (the light R1, the light G1, and thelight B1) from the pixel 47 to generate light 55 of a predeterminedcolor that is casted to the display surface side, as illustrated in FIG.7A. Thus, driving with extremely low power consumption can be performed.

[Second Mode]

FIG. 7B illustrates an example of an operation mode in which the displayelements 46R, 46G, and 46B are driven to display an image. In the casewhere, for example, the illuminance of external light is extremely low,the pixel unit 45 does not drive the pixel 47 and mixes only light (thelight R2, the light G2, and the light B2) from the pixel 46 to generatethe light 55 of a predetermined color that is emitted to the displaysurface side, as illustrated in FIG. 7B. Accordingly, a clear image canbe displayed. The luminance is lowered when the illuminance of externallight is low, which can prevent a user from feeling dazzle and reducepower consumption.

[Third Mode]

FIG. 7C illustrates an example of an operation mode in which the displayelements 47R, 47G, and 47B that reflect external light and the displayelements 46R, 46G, and 46B that emit light are driven together todisplay an image. As illustrated in FIG. 7C, the pixel unit 45 can mixlight of six colors, that is, the light R1, G1, B1, R2, G2, and B2, togenerate the light 55 of a predetermined color that is casted to thedisplay surface side.

The above is the description of the structure example of the pixel unit45.

At least part of this embodiment can be implemented in combination withany of the other embodiments and the other examples described in thisspecification as appropriate.

Embodiment 3

An example of a display panel that can be used for the display device ofone embodiment of the present invention is described below. The displaypanel described below as an example includes both a reflective liquidcrystal element and a light-emitting element and can display an image inboth a transmissive mode and a reflective mode.

Structure Example

FIG. 8A is a block diagram illustrating an example of the structure of adisplay device 400. The display device 400 includes a plurality ofpixels 410 that are arranged in a matrix in a display portion 362. Thedisplay device 400 also includes a circuit GD and a circuit SD. Inaddition, the display device 400 includes a plurality of wirings G1, aplurality of wirings G2, a plurality of wirings ANO, and a plurality ofwirings CSCOM, which are electrically connected to the circuit GD andthe plurality of pixels 410 arranged in a direction R. Moreover, thedisplay device 400 includes a plurality of wirings S1 and a plurality ofwirings S2, which are electrically connected to the circuit SD and theplurality of pixels 410 arranged in a direction C.

Although the display device includes one circuit GD and one circuit SDhere for simplification, the circuits GD and SD for driving a liquidcrystal element and the circuits GD and SD for driving a light-emittingelement may be provided separately.

The pixel 410 includes a reflective liquid crystal element and alight-emitting element. In the pixel 410, the liquid crystal element andthe light emitting element overlap with each other.

FIG. 8B1 illustrates a structure example of a conductive layer 311 bincluded in the pixel 410. The conductive layer 311 b serves as areflective electrode of the liquid crystal element in the pixel 410. Theconductive layer 311 b has an opening 451.

In FIG. 8B1, a light-emitting element 360 in a region overlapping withthe conductive layer 311 b is denoted by a dashed line. Thelight-emitting element 360 overlaps with the opening 451 of theconductive layer 311 b. Thus, light from the light-emitting element 360is emitted to a display surface side through the opening 451.

In FIG. 8B1, the pixels 410 adjacent in the direction R correspond todifferent colors. As illustrated in FIG. 8B1, the openings 451 arepreferably provided in different positions in the conductive layers 311b of the two pixels adjacent in the direction R so as not to be alignedin one line. This allows the two adjacent light-emitting elements 360 tobe apart from each other, thereby preventing light emitted by thelight-emitting element 360 from entering a coloring layer in theadjacent pixel 410 (such a phenomenon is also referred to as crosstalk).Furthermore, since the two adjacent light-emitting elements 360 can bearranged apart from each other, a high-resolution display device can beobtained even when EL layers of the light-emitting elements 360 areseparately formed with a shadow mask or the like.

Alternatively, arrangement illustrated in FIG. 8B2 may be employed.

If the ratio of the total area of the opening 451 to the total areaexcept for the opening is too large, display performed with the liquidcrystal element will be dark. If the ratio of the total area of theopening 451 to the total area except for the opening is too small,display performed with the light-emitting element 360 will be dark.

If the area of the opening 451 in the conductive layer 311 b serving asa reflective electrode is too small, light emitted from thelight-emitting element 360 cannot be efficiently extracted.

The shape of the opening 451 can be, for example, polygonal,quadrangular, elliptical, circular, or cross-shaped. Alternatively, theopening 451 may have a stripe shape, a slit shape, or a checkeredpattern. The opening 451 may be provided close to the adjacent pixel.Preferably, the opening 451 is provided close to another pixel emittinglight of the same color as that exhibited by the pixel including theopening 451, in which case crosstalk can be suppressed.

Circuit Structure Example

FIG. 9 is a circuit diagram illustrating a structure example of thepixel 410. FIG. 9 shows two adjacent pixels 410.

The pixel 410 includes a switch SW1, a capacitor C1, a liquid crystalelement 340, a switch SW2, a transistor M, a capacitor C2, thelight-emitting element 360, and the like. The pixel 410 is electricallyconnected to the wiring G1, the wiring G2, the wiring ANO, the wiringCSCOM, the wiring S1, and the wiring S2. FIG. 9 illustrates a wiringVCOM1 electrically connected to the liquid crystal element 340 and awiring VCOM2 electrically connected to the light-emitting element 360.

FIG. 9 illustrates an example in which a transistor is used as each ofthe switches SW1 and SW2.

A gate of the switch SW1 is connected to the wiring G1. One of a sourceand a drain of the switch SW1 is connected to the wiring S1, and theother of the source and the drain is connected to one electrode of thecapacitor C1 and one electrode of the liquid crystal element 340. Theother electrode of the capacitor C1 is connected to the wiring CSCOM.The other electrode of the liquid crystal element 340 is connected tothe wiring VCOM1.

A gate of the switch SW2 is connected to the wiring G2. One of a sourceand a drain of the switch SW2 is connected to the wiring S2, and theother of the source and the drain is connected to one electrode of thecapacitor C2 and a gate of the transistor M. The other electrode of thecapacitor C2 is connected to one of a source and a drain of thetransistor M and the wiring ANO. The other of the source and the drainof the transistor M is connected to one electrode of the light-emittingelement 360. The other electrode of the light-emitting element 360 isconnected to the wiring VCOM2.

FIG. 9 illustrates an example in which the transistor M includes twogates between which a semiconductor is present and which are connectedto each other. This structure can increase the amount of current flowingthrough the transistor M.

The wiring G1 can be supplied with a signal for changing the on/offstate of the switch SW1. A predetermined potential can be supplied tothe wiring VCOM1. The wiring S1 can be supplied with a signal forchanging the alignment state of a liquid crystal of the liquid crystalelement 340. A predetermined potential can be supplied to the wiringCSCOM.

The wiring G2 can be supplied with a signal for changing the on/offstate of the switch SW2. The wiring VCOM2 and the wiring ANO can besupplied with potentials having a difference large enough to make thelight-emitting element 360 emit light. The wiring S2 can be suppliedwith a signal for changing the conduction state of the transistor M.

For example, in the reflective mode, the pixel 410 of FIG. 9 can bedriven with the signals supplied to the wirings G1 and S1 to display animage with the use of the optical modulation of the liquid crystalelement 340. In the transmissive mode, the pixel can be driven with thesignals supplied to the wirings G2 and S2 to display an image with theuse of emission by the light-emitting element 360. In the case whereboth modes are performed at the same time, the pixel can be driven withthe signals to the wiring G1, the wiring G2, the wiring S1, and thewiring S2.

Although FIG. 9 illustrates an example in which one liquid crystalelement 340 and one light-emitting element 360 are provided in one pixel410, one embodiment of the present invention is not limited thereto.FIG. 10A illustrates an example in which one liquid crystal element 340and four light-emitting elements 360 (light-emitting elements 360 r, 360g, 360 b, and 360 w) are provided in one pixel 410.

The structure shown in FIG. 10A, which is similar to FIG. 9, furtherincludes a wiring G3 and a wiring S3 that are connected to the pixel410.

In the example in FIG. 10A, light-emitting elements emitting red light(R), green light (G), blue light (B), and white light (W) can be used asthe four light-emitting elements 360, for example. As the liquid crystalelement 340, a reflective liquid crystal element casting white light canbe used. Thus, in the case of performing display in the reflective mode,white display with high reflectance can be performed. In the case ofperforming display in the transmissive mode, an image can be displayedwith a high color rendering property at low power consumption.

FIG. 10B illustrates a structure example of the pixel 410. The pixel 410includes the light-emitting element 360 w that overlaps with an openingof an electrode 311, and the light-emitting elements 360 r, 360 g, and360 b that are arranged in the periphery of the electrode 311. Thelight-emitting areas of the light-emitting elements 360 r, 360 g, and360 b are preferably almost equal to one another.

Structure Example of Display Panel

FIG. 11 is a schematic perspective view of a display panel 300 of oneembodiment of the present invention. In the display panel 300, asubstrate 351 and a substrate 361 are attached to each other. In FIG.11, the substrate 361 is denoted by a dashed line.

The display panel 300 includes the display portion 362, a circuit 364, awiring 365, and the like. The substrate 351 is provided with the circuit364, the wiring 365, the conductive layer 311 b that serves as a pixelelectrode, and the like. In FIG. 11, an IC 373 and an FPC 372 aremounted on the substrate 351. Thus, the structure illustrated in FIG. 11can be referred to as a display module including the display panel 300,the FPC 372, and the IC 373.

As the circuit 364, for example, a circuit functioning as a scan linedriver circuit can be used.

The wiring 365 has a function of supplying signals and electric power tothe display portion and the circuit 364. The signal or electric power isinput to the wiring 365 from the outside through the FPC 372 or from theIC 373.

FIG. 11 shows an example in which the IC 373 is provided on thesubstrate 351 by a chip on glass (COG) method or the like. As the IC373, an IC functioning as a scan line driver circuit, a signal linedriver circuit, or the like can be used. Note that the IC 373 is notnecessarily provided when, for example, the display panel 300 includescircuits serving as a scan line driver circuit and a signal line drivercircuit or when the circuits serving as a scan line driver circuit and asignal line driver circuit are provided outside and a signal for drivingthe display panel 300 is input through the FPC 372. Alternatively, theIC 373 may be mounted on the FPC 372 by a chip on film (COF) method orthe like.

FIG. 11 also shows an enlarged view of part of the display portion 362.The conductive layers 311 b included in a plurality of display elementsare arranged in a matrix in the display portion 362. The conductivelayer 311 b has a function of reflecting visible light and serves as areflective electrode of the liquid crystal element 340 described later.

As illustrated in FIG. 11, the conductive layer 311 b has an opening.The light-emitting element 360 is positioned closer to the substrate 351than the conductive layer 311 b is. Light is emitted from thelight-emitting element 360 to the substrate 361 side through the openingin the conductive layer 311 b.

A touch sensor may be provided for the substrate 361. For example, asheet-like capacitive touch sensor 366 may be provided to overlap withthe display portion 362. Alternatively, a touch sensor may be providedbetween the substrate 361 and the substrate 351. When a touch sensor isbetween the substrates 361 and 351, an optical touch sensor using aphotoelectric conversion element can be used as well as a capacitivetouch sensor.

FIG. 12 shows an example of a capacitive touch sensor provided for asubstrate. Part of the touch sensor is enlarged in the drawing. FIG. 13Ais a top view of the touch sensor; the touch sensor includes a proximitysensor. FIG. 13B is a cross-sectional view along line X3-X4 in FIG. 13A.A substrate 560 for which the touch sensor is provided corresponds tothe second substrate 12 shown in FIGS. 1A and 1B and the like.

An insulating film 501B in FIG. 13B corresponds to the adhesive layer 26shown in FIGS. 1A and 1B and the like. An insulating film 572 includes aregion sandwiched between the insulating film 501B and a proximitysensor 575.

A sensing element for sensing a change in capacitance, illuminance,magnetic force, a radio wave, pressure, or the like caused by anapproach of an object and supplying a signal based on the sensedphysical quantity can be used for the proximity sensor 575.

For example, a conductive film, a photoelectric conversion element, amagnetic sensing element, a piezoelectric element, or a resonator can beused as the sensing element.

For example, a sensing circuit having a function of supplying a signalthat varies according to the parasitic capacitance of a conductive filmcan be used for the proximity sensor 575. A control signal is suppliedto a first electrode, and the potential, current, or the like of asecond electrode that changes according to the supplied control signaland the capacitance is obtained and can be supplied as a sensing signal.Thus, a finger or the like that approaches the conductive film in theair can be sensed with change in capacitance.

For example, the proximity sensor 575 can include a first electrodeC1(g) and a second electrode C2(h) (see FIG. 12 and FIG. 13A). Thesecond electrode C2(h) has a portion not overlapping with the firstelectrode C1(g). Note that each of g and h is a natural number of 1 ormore.

Specifically, in the proximity sensor 575, the first electrode C1(g) iselectrically connected to a control line CL(g) extended in a rowdirection (a direction indicated by the arrow R in FIG. 12) and thesecond electrode C2(h) is electrically connected to a signal line ML(h)extended in a column direction intersecting with the row direction (adirection indicated by the arrow C in FIG. 12).

For the first electrode C1(g) or the second electrode C2(h), aconductive film whose light-transmitting regions overlap with the pixel410 can be used, for example.

For the first electrode C1(g) or the second electrode C2(h), a net-likeconductive film whose openings 576 overlap with the pixel 410 can beused, for example.

The control line CL(g) is provided with the wiring BR(g, h). The controlline CL(g) intersects with the signal line ML(h) in a portion where thewiring BR(g, h) exists (see FIG. 13B).

A stacked-layer film can be used for the first electrode C1(g), thesecond electrode C2(h), the control line CL(g), or the signal lineML(h), for example. Specifically, a stacked-layer film in which aconductive film CL(g)A and a dark-colored film CL(g)B are stacked sothat the conductive film CL(g)A is sandwiched between the dark-coloredfilm CL(g)B and the pixel 410 can be used.

A film having a lower visible light reflectance than the conductive filmCL(g)A can be used as the dark-colored film CL(g)B, for example. Thus,reflection of visible light due to the first electrode C1(g), the secondelectrode C2(h), the control line CL(g), or the signal line ML(h) can bereduced. Consequently, display of the display portion 362 can beclearer, so that favorable display can be obtained. In addition, thethickness of the display device can be reduced. Moreover, stress causedin the substrate 560 or the like when the display device is bent can bereduced.

For example, a material available for the wirings G1, G2, ANO, CSCOM,and the like can be used for the conductive film CL(g)A.

A film containing cupric oxide or a film containing copper chloride ortellurium chloride can be used as the dark-colored film CL(g)B, forexample. Alternatively, the dark-colored film CL(g)B may be formed witha metal particle such as an Ag particle, an Ag fiber, or a Cu particle,a carbon nanoparticle such as a carbon nanotube (CNT) or graphene, or aconductive high molecule such as PEDOT, polyaniline, or polypyrrole, forexample.

The proximity sensor 575 includes an insulating film 571 between thewiring BR(g, h) and the signal line ML(h). Thus, a short circuit betweenthe wiring BR(g, h) and the signal line ML(h) can be prevented.

Example of Cross-Sectional Structure

FIG. 14 shows an example of cross sections of part of a region includingthe FPC 372, part of a region including the circuit 364, part of aregion including the display portion 362, and the touch sensor 366 ofthe display panel illustrated in FIG. 11.

The display panel includes an insulating layer 220 between thesubstrates 351 and 361. The display panel also includes thelight-emitting element 360, a transistor 201, a transistor 205, atransistor 206, a coloring layer 134, and the like between the substrate351 and the insulating layer 220. Furthermore, the display panelincludes the liquid crystal element 340, a coloring layer 131, and thelike between the insulating layer 220 and the substrate 361. Thesubstrate 361 and the insulating layer 220 are bonded with an adhesivelayer 141. The substrate 351 and the insulating layer 220 are bondedwith an adhesive layer 142.

The transistor 206 is electrically connected to the liquid crystalelement 340. The transistor 205 is electrically connected to thelight-emitting element 360. Since the transistors 205 and 206 are formedon a surface of the insulating layer 220 that is on the substrate 351side, the transistors 205 and 206 can be formed through the sameprocess.

The substrate 361 is provided with the coloring layer 131, alight-blocking layer 132, an insulating layer 121, a conductive layer313 serving as a common electrode of the liquid crystal element 340, analignment film 133 b, an insulating layer 117, and the like. Theinsulating layer 117 serves as a spacer for holding a cell gap of theliquid crystal element 340.

Insulating layers such as an insulating layer 211, an insulating layer212, an insulating layer 213, an insulating layer 214, and an insulatinglayer 215 are provided on the substrate 351 side of the insulating layer220. Parts of the insulating layer 211 function as gate insulatinglayers of the transistors. The insulating layers 212, 213, and 214 coverthe transistors. The insulating layer 215 covers the insulating layer214. The insulating layers 214 and 215 each function as a planarizationlayer. Note that the three insulating layers, the insulating layers 212,213, and 214, are provided to cover the transistors and the like in thisexample; however, one embodiment of the present invention is not limitedto this example, and four or more insulating layers, a single insulatinglayer, or two insulating layers may be provided. The insulating layer214 functioning as a planarization layer is not necessarily providedwhen not needed.

The transistors 201, 205, and 206 each include a conductive layer 221part of which functions as a gate, conductive layers 222 parts of whichfunction as a source and a drain, and a semiconductor layer 231. Here, aplurality of layers obtained by processing the same conductive film areshown with the same hatching pattern.

The liquid crystal element 340 is a reflective liquid crystal element.The liquid crystal element 340 has a structure in which a conductivelayer 311 a, a liquid crystal 312, and the conductive layer 313 arestacked. The conductive layer 311 b that reflects visible light is incontact with the substrate 351 side of the conductive layer 311 a. Theconductive layer 311 b includes an opening 251. The conductive layers311 a and 313 each contain a material transmitting visible light. Inaddition, an alignment film 133 a is between the liquid crystal 312 andthe conductive layer 311 a and the alignment film 133 b is between theliquid crystal 312 and the conductive layer 313.

A light diffusion plate 129 and a polarizing plate 130 are provided foran outer surface of the substrate 361. As the polarizing plate 130, alinear polarizing plate or a circularly polarizing plate can be used. Anexample of a circularly polarizing plate is a stack including a linearpolarizing plate and a quarter-wave retardation plate. Such a structurecan reduce reflection of external light. The light diffusion plate canreduce reflection of external light. The cell gap, alignment, drivevoltage, and the like of the liquid crystal element used as the liquidcrystal element 340 are controlled depending on the kind of thepolarizing plate so that desirable contrast is obtained.

In the liquid crystal element 340, the conductive layer 311 b has afunction of reflecting visible light, and the conductive layer 313 has afunction of transmitting visible light. Light entering from thesubstrate 361 side is polarized by the polarizing plate 130, passesthrough the conductive layer 313 and the liquid crystal 312, and isreflected by the conductive layer 311 b. Then, the light passes throughthe liquid crystal 312 and the conductive layer 313 again and reachesthe polarizing plate 130. Here, the alignment of the liquid crystal iscontrolled by voltage applied between the conductive layer 311 b and theconductive layer 313, whereby optical modulation of light can becontrolled. That is, the intensity of light casted through thepolarizing plate 130 can be controlled. Light other than one in aparticular wavelength region is absorbed by the coloring layer 131, andthus, extracted light exhibits, for example, red light.

The light-emitting element 360 is a bottom-emission light-emittingelement. The light-emitting element 360 has a structure in which aconductive layer 191, an EL layer 192, and a conductive layer 193 b arestacked in this order from the insulating layer 220 side. In addition, aconductive layer 193 a covers the conductive layer 193 b. The conductivelayer 193 b contains a material reflecting visible light, and theconductive layers 191 and 193 a each contain a material transmittingvisible light. Light is emitted from the light-emitting element 360 tothe substrate 361 side through the coloring layer 134, the insulatinglayer 220, the opening 251, the conductive layer 313, and the like.

Here, as illustrated in FIG. 14, the conductive layer 311 a transmittingvisible light is preferably provided for the opening 251. Accordingly,the liquid crystal 312 is aligned in a region overlapping with theopening 251 as in the other regions, which prevents an alignment defectof the liquid crystal in the boundary portion of these regions andreduces undesired light leakage.

An insulating layer 217 is provided on the insulating layer 216 coveringan end portion of the conductive layer 191. The insulating layer 217 hasa function as a spacer for preventing the insulating layer 220 and thesubstrate 351 from getting closer than necessary. In addition, in thecase where the EL layer 192 or the conductive layer 193 a is formedusing a blocking mask (metal mask), the insulating layer 217 may have afunction of preventing the blocking mask from being in contact with asurface on which the EL layer 192 or the conductive layer 193 a isformed. Note that the insulating layer 217 is not necessarily providedwhen not needed.

One of a source and a drain of the transistor 205 is electricallyconnected to the conductive layer 191 of the light-emitting element 360through a conductive layer 224.

One of a source and a drain of the transistor 206 is electricallyconnected to the conductive layer 311 b through a connection portion207. The conductive layers 311 b and 311 a are in contact with andelectrically connected to each other. Here, in the connection portion207, the conductive layers on both surfaces of the insulating layer 220are connected to each other in an opening in the insulating layer 220.

A connection portion 204 is provided in a region in which the substrate351 and the substrate 361 do not overlap with each other. The connectionportion 204 is electrically connected to the FPC 372 through aconnection layer 242. The connection portion 204 has a structure similarto that of the connection portion 207. On the surface of the connectionportion 204, a conductive layer obtained by processing the sameconductive film as the conductive layer 311 a is exposed. Thus, theconnection portion 204 and the FPC 372 can be electrically connected toeach other through the connection layer 242.

A connection portion 252 is provided in part of a region where theadhesive layer 141 is provided. In the connection portion 252, theconductive layer obtained by processing the same conductive film as theconductive layer 311 a is electrically connected to part of theconductive layer 313 with a connector 243. Accordingly, a signal or apotential input from the FPC 372 connected to the substrate 351 side canbe supplied to the conductive layer 313 formed on the substrate 361 sidethrough the connection portion 252.

As the connector 243, a conductive particle can be used, for example. Asthe conductive particle, a particle of an organic resin, silica, or thelike coated with a metal material can be used. It is preferable to usenickel or gold, which can reduce contact resistance, as the metalmaterial. It is also preferable to use a particle coated with layers oftwo or more kinds of metal materials, such as a particle coated withnickel and further with gold. As the connector 243, a material capableof elastic deformation or plastic deformation is preferably used. Asillustrated in FIG. 14, the connector 243 that is the conductiveparticle has a shape that is vertically crushed in some cases. With thecrushed shape, the contact area between the connector 243 and aconductive layer electrically connected to the connector 243 can beincreased, thereby reducing contact resistance and suppressing thegeneration of problems such as disconnection.

The connector 243 is preferably covered with the adhesive layer 141. Forexample, the connectors 243 are dispersed in the adhesive layer 141 thatis not yet cured.

The touch sensor 366 provided for the substrate 560 is attached to thepolarizing plate 130 with the adhesive layer 141 therebetween.

FIG. 14 illustrates an example where the circuit 364 includes thetransistor 201.

In the example of FIG. 14, the transistors 201 and 205 each have astructure in which the semiconductor layer 231 where a channel is formedis between two gates. One gate is formed by the conductive layer 221 andthe other gate is formed by a conductive layer 223 overlapping with thesemiconductor layer 231 with the insulating layer 212 therebetween. Sucha structure enables control of threshold voltages of the transistor. Inthat case, the two gates may be connected to each other and suppliedwith the same signal to operate the transistor. Such a transistor canhave a higher field-effect mobility and thus have higher on-statecurrent than other transistors. Consequently, a circuit capable ofhigh-speed operation can be obtained. Furthermore, the area occupied bya circuit portion can be reduced. The use of the transistor having highon-state current can reduce signal delay in wirings and can reducedisplay unevenness even in a display panel with the increased number ofwirings due to increase in size or definition.

Note that the transistor included in the circuit 364 and the transistorincluded in the display portion 362 may have the same structure. Aplurality of transistors included in the circuit 364 may have the samestructure or different structures. A plurality of transistors includedin the display portion 362 may have the same structure or differentstructures.

A material through which impurities such as water or hydrogen do noteasily diffuse is preferably used for at least one of the insulatinglayers 212 and 213 that cover the transistors. Thus, the insulatinglayer 212 or the insulating layer 213 can function as a barrier film.Such a structure can effectively suppress diffusion of the impuritiesinto the transistors from the outside, and a highly reliable displaypanel can be provided.

The insulating layer 121 is provided on the substrate 361 side to coverthe coloring layer 131 and the light-blocking layer 132. The insulatinglayer 121 may have a function as a planarization layer. The insulatinglayer 121 enables the conductive layer 313 to have an almost flatsurface, resulting in a uniform alignment state of the liquid crystal312.

[Components]

The above components will be described below.

[Substrate]

A material having a flat surface can be used as the substrate includedin the display panel. The substrate through which light from the displayelement is extracted is formed using a material that transmits thelight. For example, a material such as glass, quartz, ceramic, sapphire,or an organic resin can be used.

The weight and thickness of the display panel can be decreased by usinga thin substrate. A flexible display panel can be obtained by using asubstrate that is thin enough to have flexibility.

Since the substrate through which light is not extracted does not needto have a light-transmitting property, a metal substrate or the like canbe used as well as the above-mentioned substrates. A metal material,which has high thermal conductivity, is preferable because it can easilyconduct heat to the whole substrate and accordingly can prevent a localtemperature rise in the display panel. To obtain flexibility orbendability, the thickness of a metal substrate is preferably greaterthan or equal to 10 and less than or equal to 200 μm, more preferablygreater than or equal to 20 μm and less than or equal to 50 μm.

Although there is no particular limitation on a material of a metalsubstrate, it is favorable to use, for example, a metal such asaluminum, copper, or nickel, or an alloy such as an aluminum alloy orstainless steel.

It is preferable to use a substrate subjected to insulation treatment,e.g., a metal substrate whose surface is oxidized or provided with aninsulating film. The insulating film may be formed by, for example, acoating method such as a spin-coating method or a dipping method, anelectrodeposition method, an evaporation method, or a sputtering method.An oxide film may be formed on the substrate surface by exposure to orheating in an oxygen atmosphere or by an anodic oxidation method or thelike.

Examples of a material that has flexibility and transmits visible lightinclude polyester resins such as polyethylene terephthalate (PET) andpolyethylene naphthalate (PEN), a polyacrylonitrile resin, a polyimideresin, a polymethyl methacrylate resin, a polycarbonate (PC) resin, apolyethersulfone (PES) resin, a polyamide resin, a cycloolefin resin, apolystyrene resin, a polyamide imide resin, a polyvinyl chloride resin,and a polytetrafluoroethylene (PTFE). In particular, a material with alow thermal expansion coefficient is preferred, and for example, apolyamide imide resin, a polyimide resin, or PET with a thermalexpansion coefficient of 30×10⁻⁶/K or less can be suitably used. Asubstrate in which a glass fiber is impregnated with an organic resin ora substrate whose thermal expansion coefficient is reduced by mixing anorganic resin with an inorganic filler can also be used. A substrateusing such a material is light in weight, and thus a display panel usingthis substrate can also be light in weight.

In the case where a fibrous body is included in the above material, ahigh-strength fiber of an organic compound or an inorganic compound isused as the fibrous body. The high-strength fiber is specifically afiber with a high tensile elastic modulus or a fiber with a high Young'smodulus. Typical examples thereof include a polyvinyl alcohol basedfiber, a polyester based fiber, a polyamide based fiber, a polyethylenebased fiber, an aramid based fiber, a polyparaphenylene benzobisoxazolefiber, a glass fiber, and a carbon fiber. As the glass fiber, a glassfiber using E glass, S glass, D glass, Q glass, or the like can begiven. These fibers may be used in a state of a woven or nonwovenfabric, and a structure body formed by impregnating the fibrous bodywith a resin and curing the resin may be used as the flexible substrate.The structure body including the fibrous body and the resin ispreferably used as the flexible substrate, in which case the reliabilityagainst breaking due to bending or local pressure can be increased.

Alternatively, glass, metal, or the like that is thin enough to haveflexibility can be used as the substrate. Alternatively, a compositematerial where glass and a resin material are attached to each otherwith an adhesive layer may be used.

A hard coat layer (e.g., a silicon nitride layer or an aluminum oxidelayer) by which a surface of a display panel is protected from damage, alayer (e.g., an aramid resin layer) that can disperse pressure, or thelike may be stacked over the flexible substrate. Furthermore, tosuppress a decrease in lifetime of the display element due to moistureand the like, an insulating film with a low water permeability may bestacked over the flexible substrate. For example, an inorganicinsulating material such as silicon nitride, silicon oxynitride, siliconnitride oxide, aluminum oxide, or aluminum nitride can be used.

The substrate may be formed by stacking a plurality of layers. When aglass layer is included, a barrier property against water and oxygen canbe improved and thus a highly reliable display panel can be provided.

[Transistor]

The transistors each include the conductive layer functioning as a gateelectrode, the semiconductor layer, the conductive layer functioning asa source electrode, the conductive layer functioning as a drainelectrode, and the insulating layer functioning as a gate insulatinglayer. In the above, a bottom-gate transistor is used.

Note that there is no particular limitation on the structure of thetransistor included in the display device of one embodiment of thepresent invention. For example, a planar transistor, a staggeredtransistor, or an inverted staggered transistor may be used. A top-gatetransistor or a bottom-gate transistor may be used. Gate electrodes maybe provided above and below a channel.

There is no particular limitation on the crystallinity of asemiconductor material used for the transistors, and an amorphoussemiconductor or a semiconductor having crystallinity (amicrocrystalline semiconductor, a polycrystalline semiconductor, asingle-crystal semiconductor, or a semiconductor partly includingcrystal regions) may be used. A semiconductor having crystallinity ispreferably used, in which case deterioration of the transistorcharacteristics can be suppressed.

As a semiconductor material used for the transistors, a metal oxidewhose energy gap is greater than or equal to 2 eV, preferably greaterthan or equal to 2.5 eV, further preferably greater than or equal to 3eV can be used. A typical example thereof is an oxide semiconductorcontaining indium, and for example, a CAC-OS described later or the likecan be used.

A transistor with an oxide semiconductor having a larger band gap and alower carrier density than silicon has a low off-state current andtherefore charges accumulated in a capacitor that is series-connected tothe transistor can be held for a long time.

The semiconductor layer can be, for example, a film represented by anIn-M-Zn-based oxide, which contains at least indium, zinc, and M (ametal such as aluminum, titanium, gallium, germanium, yttrium,zirconium, lanthanum, cerium, tin, neodymium, or hafnium).

In the case where the oxide semiconductor in the semiconductor layercontains an In-M-Zn-based oxide, it is preferable that the atomic ratioof metal elements of a sputtering target used for forming a film of theIn-M-Zn oxide satisfy In M and Zn M. The atomic ratio of metal elementsin such a sputtering target is preferably, for example, In:M:Zn=1:1:1,In:M:Zn=1:1:1.2, In:M:Zn=3:1:2, In:M:Zn=4:2:3, In:M:Zn=4:2:4.1,In:M:Zn=5:1:6, In:M:Zn=5:1:7, or In:M:Zn=5:1:8. Note that the atomicratio of metal elements in the formed oxide semiconductor layer variesfrom the above atomic ratios of metal elements of the sputtering targetsin a range of ±40%.

The bottom-gate transistor described in this embodiment is preferable toreduce the number of manufacturing steps. When an oxide semiconductor,which can be formed at a lower temperature than polycrystalline silicon,is used, materials with low heat resistance can be used for a wiring, anelectrode, or a substrate below the semiconductor layer, so that therange of choices of materials can be widened. For example, an extremelylarge glass substrate can be favorably used.

An oxide semiconductor film with a low carrier density is used as thesemiconductor layer. For example, the semiconductor layer is an oxidesemiconductor film whose carrier density is lower than or equal to1×10¹⁷/cm³, preferably lower than or equal to 1×10¹⁵/cm³, furtherpreferably lower than or equal to 1×10¹³/cm³, still further preferablylower than or equal to 1×10¹¹/cm³, even further preferably lower than1×10¹⁰/cm³, and higher than or equal to 1×10⁻⁹/cm³. Such an oxidesemiconductor is referred to as a highly purified intrinsic orsubstantially highly purified intrinsic oxide semiconductor. The oxidesemiconductor has a low impurity concentration and a low density ofdefect states and thus can be referred to as an oxide semiconductorhaving stable characteristics.

However, the composition is not limited to those described above, and amaterial having the appropriate composition may be used depending onrequired semiconductor characteristics and electrical characteristics ofthe transistor (e.g., field-effect mobility and threshold voltage). Toobtain the required semiconductor characteristics of the transistor, itis preferable that the carrier density, the impurity concentration, thedefect density, the atomic ratio between a metal element and oxygen, theinteratomic distance, the density, and the like of the semiconductorlayer be set to appropriate values.

When silicon or carbon that is one of elements belonging to Group 14 iscontained in the oxide semiconductor included in the semiconductorlayer, the semiconductor layer includes an increased number of oxygenvacancies, and thus becomes n-type. Thus, the concentration of siliconor carbon (measured by secondary ion mass spectrometry) in thesemiconductor layer is set to lower than or equal to 2×10¹⁸ atoms/cm³,preferably lower than or equal to 2×10¹⁷ atoms/cm³.

Alkali metal and alkaline earth metal can generate carriers when bondedto the oxide semiconductor, which could increase the off-state currentof the transistor. Therefore, the concentration of alkali metal oralkaline earth metal in the semiconductor layer, which is measured bysecondary ion mass spectrometry, is set to lower than or equal to 1×10¹⁸atoms/cm³, preferably lower than or equal to 2×10¹⁶ atoms/cm³.

When nitrogen is contained in the oxide semiconductor included in thesemiconductor layer, electrons serving as carriers are generated and thecarrier density increases, so that the semiconductor layer easilybecomes n-type. Thus, a transistor including the oxide semiconductorthat contains nitrogen is likely to be normally-on. Hence, theconcentration of nitrogen in the semiconductor layer, which is measuredby secondary ion mass spectrometry, is preferably set to lower than orequal to 5×10¹⁸ atoms/cm³.

The semiconductor layer may have a non-single-crystal structure, forexample. The non-single-crystal structure includes CAAC-OS (c-axisaligned crystalline oxide semiconductor, or c-axis aligneda-b-plane-anchored crystalline oxide semiconductor) including a c-axisaligned crystal, a polycrystalline structure, a microcrystallinestructure, and an amorphous structure, for example. Among thenon-single-crystal structures, the amorphous structure has the highestdensity of defect states, whereas CAAC-OS has the lowest density ofdefect states.

An oxide semiconductor film having an amorphous structure has, forexample, disordered atomic arrangement and no crystalline component. Anoxide film having an amorphous structure has, for example, an absolutelyamorphous structure and no crystal part.

Note that the semiconductor layer may be a mixed film including two ormore of the following: a region having an amorphous structure, a regionhaving a microcrystalline structure, a region having a polycrystallinestructure, a region of CAAC-OS, and a region having a single-crystalstructure. The mixed film has, for example, a single-layer structure ora stacked-layer structure including two or more of the above regions.

<Composition of CAC-OS>

A metal oxide having a CAC composition that can be used for thesemiconductor layer of the transistor disclosed in one embodiment of thepresent invention will be described in detail below. Here, descriptionis made using a CAC-OS as a typical example of the metal oxide having aCAC composition.

For example, in the CAC-OS, elements included in the metal oxide areunevenly distributed, and regions 101 mainly including an element andregions 102 mainly including another element are formed, as shown in theexample of FIG. 15 in which CAC-OS is formed over an insulating film106. The regions 101 and 102 are mixed or distributed to form a mosaicpattern. In other words, the CAC-OS has a composition in which elementsincluded in a metal oxide are unevenly distributed. Materials includingunevenly distributed elements each have a size of greater than or equalto 0.5 nm and less than or equal to 10 nm, preferably greater than orequal to 0.5 nm and less than or equal to 3 nm, or a similar size.

The physical properties of a region including an unevenly distributedelement are determined by the properties of the element. For example, aregion including an unevenly distributed element which relatively tendsto serve as an insulator among elements included in a metal oxide servesas a dielectric region. In contrast, a region including an unevenlydistributed element which relatively tends to serve as a conductor amongelements included in a metal oxide serves as a conductive region. Amaterial in which conductive regions and dielectric regions are mixed toform a mosaic pattern serves as a semiconductor.

That is, a metal oxide in one embodiment of the present invention is akind of matrix composite or metal matrix composite, in which materialshaving different physical properties are mixed.

Note that an oxide semiconductor preferably contains at least indium. Inparticular, indium and zinc are preferably contained. In addition, anelement M (M is one or more of gallium, aluminum, silicon, boron,yttrium, copper, vanadium, beryllium, titanium, iron, nickel, germanium,zirconium, molybdenum, lanthanum, cerium, neodymium, hafnium, tantalum,tungsten, magnesium, and the like) may be contained.

For example, of the CAC-OS, an In—Ga—Zn oxide with the CAC composition(such an In—Ga—Zn oxide may be particularly referred to as CAC-IGZO) hasa composition in which materials are separated into indium oxide(InO_(X1), where X1 is a real number greater than 0) or indium zincoxide (In_(X2)Zn_(Y2)O_(Z2), where X2, Y2, and Z2 are real numbersgreater than 0), and gallium oxide (GaO_(X3), where X3 is a real numbergreater than 0) or gallium zinc oxide (Ga_(X4)Zn_(Y4)O_(Z4), where X4,Y4, and Z4 are real numbers greater than 0), and a mosaic pattern isformed. Then, InO_(X1) or In_(X2)Zn_(Y2)O_(Z2) forming the mosaicpattern is evenly distributed in the film. This composition is alsoreferred to as a cloud-like composition.

That is, the CAC-OS is a composite oxide semiconductor with acomposition in which a region including GaO_(x3) as a main component anda region including In_(X2)Zn_(Y2)O_(Z2) or InO_(X1) as a main componentare mixed. Note that in this specification, for example, when the atomicratio of In to an element Min a first region is greater than the atomicratio of In to an element M in a second region, the first region hashigher In concentration than the second region.

Note that a compound including In, Ga, Zn, and O is also known as IGZO.Typical examples of IGZO include a crystalline compound represented byInGaO₃(ZnO)_(m1) (m1 is a natural number) and a crystalline compoundrepresented by In_((1+x0))Ga_((1−x0))O₃(ZnO)_(m0) (−1≤x0≤1; m0 is agiven number).

The above crystalline compounds have a single crystal structure, apolycrystalline structure, or a CAAC structure. Note that the CAACstructure is a crystal structure in which a plurality of IGZOnanocrystals have c-axis alignment and are connected in the a-b planedirection without alignment.

On the other hand, the CAC-OS relates to the material composition of anoxide semiconductor. In a material composition of a CAC-OS including In,Ga, Zn, and O, nanoparticle regions including Ga as a main component areobserved in part of the CAC-OS and nanoparticle regions including In asa main component are observed in part thereof. These nanoparticleregions are randomly dispersed to form a mosaic pattern. Therefore, thecrystal structure is a secondary element for the CAC-OS.

Note that in the CAC-OS, a stacked-layer structure including two or morefilms with different atomic ratios is not included. For example, atwo-layer structure of a film including In as a main component and afilm including Ga as a main component is not included.

A boundary between the region including GaO_(X3) as a main component andthe region including In_(X2)Zn_(Y2)O_(Z2) or InO_(X1) as a maincomponent is not clearly observed in some cases.

In the case where one or more of aluminum, silicon, boron, yttrium,copper, vanadium, beryllium, titanium, iron, nickel, germanium,zirconium, molybdenum, lanthanum, cerium, neodymium, hafnium, tantalum,tungsten, magnesium, and the like are contained instead of gallium in aCAC-OS, nanoparticle regions including the selected element(s) as a maincomponent(s) are observed in part of the CAC-OS and nanoparticle regionsincluding In as a main component are observed in part thereof, and thesenanoparticle regions are randomly dispersed to form a mosaic pattern inthe CAC-OS.

<Analysis of CAC-OS>

Next, measurement results of an oxide semiconductor over a substrate bya variety of methods are described.

<<Structure of Samples and Formation Method Thereof>>

Nine samples of one embodiment of the present invention are describedbelow. The samples are formed at different substrate temperatures andwith different ratios of an oxygen gas flow rate in formation of theoxide semiconductor. Note that each sample includes a substrate and anoxide semiconductor over the substrate.

A method for forming the samples is described.

A glass substrate is used as the substrate. Over the glass substrate, a100-nm-thick In—Ga—Zn oxide is formed as an oxide semiconductor with asputtering apparatus. The formation conditions are as follows: thepressure in a chamber is 0.6 Pa, and an oxide target (with an atomicratio of In:Ga:Zn=4:2:4.1) is used as a target. The oxide targetprovided in the sputtering apparatus is supplied with an AC power of2500 W.

As for the conditions in the formation of the oxide of the nine samples,the substrate temperature is set to a temperature that is not increasedby intentional heating (hereinafter such a temperature is also referredto as room temperature or R.T.), to 130° C., and to 170° C. The ratio ofa flow rate of an oxygen gas to a flow rate of a mixed gas of Ar andoxygen (also referred to as an oxygen gas flow rate ratio) is set to10%, 30%, and 100%.

<<Analysis by X-Ray Diffraction>>

In this section, results of X-ray diffraction (XRD) measurementperformed on the nine samples are described. As an XRD apparatus, D8ADVANCE manufactured by Bruker AXS is used. The conditions are asfollows: scanning is performed by an out-of-plane method at θ/2θ, thescanning range is 15 deg. to 50 deg., the step width is 0.02 deg., andthe scanning speed is 3.0 deg./min.

FIG. 16 shows XRD spectra measured by an out-of-plane method. In FIG.16, the top row shows the measurement results of the samples formed at asubstrate temperature of 170° C.; the middle row shows the measurementresults of the samples formed at a substrate temperature of 130° C.; thebottom row shows the measurement results of the samples formed at asubstrate temperature of R.T. The left column shows the measurementresults of the samples formed with an oxygen gas flow rate ratio of 10%;the middle column shows the measurement results of the samples formedwith an oxygen gas flow rate ratio of 30%; the right column shows themeasurement results of the samples formed with an oxygen gas flow rateratio of 100%.

In the XRD spectra shown in FIG. 16, the higher the substratetemperature at the time of formation is or the higher the oxygen gasflow rate ratio at the time of formation is, the higher the intensity ofthe peak at around 2q=31° is. Note that it is found that the peak ataround 20=31° is derived from a crystalline IGZO compound whose c-axesare aligned in a direction substantially perpendicular to a formationsurface or a top surface of the crystalline IGZO compound (such acompound is also referred to as c-axis aligned crystalline (CAAC) IGZO).

As shown in the XRD spectra in FIG. 16, as the substrate temperature atthe time of formation is lower or the oxygen gas flow rate ratio at thetime of formation is lower, a peak becomes less clear. Accordingly, itis found that there are no alignment in the a-b plane direction andc-axis alignment in the measured areas of the samples that are formed ata lower substrate temperature or with a lower oxygen gas flow rateratio.

<<Analysis with Electron Microscope>>

This section describes the observation and analysis results of thesamples formed at a substrate temperature of R.T. and with an oxygen gasflow rate ratio of 10% with a high-angle annular dark-field scanningtransmission electron microscope (HAADF-STEM). An image obtained with anHAADF-STEM is also referred to as a TEM image.

Described are the results of image analysis of plan-view images andcross-sectional images obtained with an HAADF-STEM (also referred to asplan-view TEM images and cross-sectional TEM images, respectively). TheTEM images are observed with a spherical aberration corrector function.The HAADF-STEM images are obtained using an atomic resolution analyticalelectron microscope JEM-ARM200F manufactured by JEOL Ltd. under thefollowing conditions: the acceleration voltage is 200 kV, andirradiation with an electron beam with a diameter of approximately 0.1nm is performed.

FIG. 17A is a plan-view TEM image of the sample fat Hied at a substratetemperature of R.T. and an oxygen gas flow rate ratio of 10%. FIG. 17Bis a cross-sectional TEM image of the sample formed at a substratetemperature of R.T. and with an oxygen gas flow rate ratio of 10%.

<<Analysis of Electron Diffraction Patterns>>

This section describes electron diffraction patterns obtained byirradiation of the sample formed at a substrate temperature of R.T. andan oxygen gas flow rate ratio of 10% with an electron beam with a probediameter of 1 nm (also referred to as a nanobeam).

Electron diffraction patterns of points indicated by black dots a1, a2,a3, a4, and a5 in the plan-view TEM image in FIG. 17A of the sampleformed at a substrate temperature of R.T. and an oxygen gas flow rateratio of 10% are observed. Note that the electron diffraction patternsare observed while electron beam irradiation is performed at a constantrate for 35 seconds. FIGS. 17C, 17D, 17E, 17F, and 17G show the resultsof the points indicated by the black dots a1, a2, a3, a4, and a5,respectively.

In FIGS. 17C, 17D, 17E, 17F, and 17G, regions with high luminance in acircular (ring) pattern can be shown. Furthermore, a plurality of spotscan be shown in a ring-like shape.

Electron diffraction patterns of points indicated by black dots b1, b2,b3, b4, and b5 in the cross-sectional TEM image in FIG. 17B of thesample Ruined at a substrate temperature of R.T. and an oxygen gas flowrate ratio of 10% are observed. FIGS. 17H, 17I, 17J, 17K, and 17L showthe results of the points indicated by the black dots b1, b2, b3, b4,and b5, respectively.

In FIGS. 17H, 17I, 17J, 17K, and 17L, regions with high luminance in aring pattern can be shown. Furthermore, a plurality of spots can beshown in a ring-like shape.

For example, when an electron beam with a probe diameter of 300 nm isincident on a CAAC-OS including an InGaZnO₄ crystal in a directionparallel to the sample surface, a diffraction pattern including a spotderived from the (009) plane of the InGaZnO₄ crystal is obtained. Thatis, the CAAC-OS has c-axis alignment and the c-axes are aligned in thedirection substantially perpendicular to the formation surface or thetop surface of the CAAC-OS. Meanwhile, a ring-like diffraction patternis shown when an electron beam with a probe diameter of 300 nm isincident on the same sample in a direction perpendicular to the samplesurface. That is, it is found that the CAAC-OS has neither a-axisalignment nor b-axis alignment.

Furthermore, a diffraction pattern like a halo pattern is observed whenan oxide semiconductor including a nanocrystal (a nanocrystalline oxidesemiconductor (nc-OS)) is subjected to electron diffraction using anelectron beam with a large probe diameter (e.g., 50 nm or larger).Meanwhile, bright spots are shown in a nanobeam electron diffractionpattern of the nc-OS obtained using an electron beam with a small probediameter (e.g., smaller than 50 nm). Furthermore, in a nanobeam electrondiffraction pattern of the nc-OS, regions with high luminance in acircular (ring) pattern are shown in some cases. Also in a nanobeamelectron diffraction pattern of the nc-OS, a plurality of bright spotsare shown in a ring-like shape in some cases.

The electron diffraction pattern of the sample formed at a substratetemperature of R.T. and with an oxygen gas flow rate ratio of 10% hasregions with high luminance in a ring pattern and a plurality of brightspots appear in the ring-like pattern. Accordingly, the sample formed ata substrate temperature of R.T. and with an oxygen gas flow rate ratioof 10% exhibits an electron diffraction pattern similar to that of thenc-OS and does not show alignment in the plane direction and thecross-sectional direction.

According to what is described above, an oxide semiconductor formed at alow substrate temperature or with a low oxygen gas flow rate ratio islikely to have characteristics distinctly different from those of anoxide semiconductor film having an amorphous structure and an oxidesemiconductor film having a single crystal structure.

<<Elementary Analysis>>

This section describes the analysis results of elements included in thesample formed at a substrate temperature of R.T. and with an oxygen gasflow rate ratio of 10%. For the analysis, by energy dispersive X-rayspectroscopy (EDX), EDX mapping images are obtained. An energydispersive X-ray spectrometer AnalysisStation JED-2300T manufactured byJEOL Ltd. is used as an elementary analysis apparatus in the EDXmeasurement. A Si drift detector is used to detect an X-ray emitted fromthe sample.

In the EDX measurement, an EDX spectrum of a point is obtained in such amanner that electron beam irradiation is performed on the point in adetection target region of a sample, and the energy of characteristicX-ray of the sample generated by the irradiation and its frequency aremeasured. In this embodiment, peaks of an EDX spectrum of the point areattributed to electron transition to the L shell in an In atom, electrontransition to the K shell in a Ga atom, and electron transition to the Kshell in a Zn atom and the K shell in an O atom, and the proportions ofthe atoms in the point are calculated. An EDX mapping image indicatingdistributions of proportions of atoms can be obtained through theprocess in an analysis target region of a sample.

FIGS. 18A to 18C show EDX mapping images in a cross section of thesample formed at a substrate temperature of R.T. and with an oxygen gasflow rate ratio of 10%. FIG. 18A shows an EDX mapping image of Ga atoms.The proportion of the Ga atoms in all the atoms is 1.18 atomic % to18.64 atomic %. FIG. 18B shows an EDX mapping image of In atoms. Theproportion of the In atoms in all the atoms is 9.28 atomic % to 33.74atomic %. FIG. 18C shows an EDX mapping image of Zn atoms. Theproportion of the Zn atoms in all the atoms is 6.69 atomic % to 24.99atomic %. FIGS. 18A to 18C show the same region in the cross section ofthe sample formed at a substrate temperature of R.T. and with an oxygengas flow rate ratio of 10%. In the EDX mapping images, the proportion ofan element is indicated by grayscale: the more measured atoms exist in aregion, the brighter the region is; the less measured atoms exist in aregion, the darker the region is. The magnification of the EDX mappingimages in FIGS. 18A to 18C is 7200000 times.

The EDX mapping images in FIGS. 18A to 18C show relative distribution ofbrightness indicating that each element has a distribution in the sampleformed at a substrate temperature of R.T. and with an oxygen gas flowrate ratio of 10%. Areas surrounded by solid lines and areas surroundedby dashed lines in FIGS. 18A to 18C are examined.

In FIG. 18A, a relatively dark region occupies a large area in the areasurrounded by the solid line, while a relatively bright region occupiesa large area in the area surrounded by the dashed line. In FIG. 18B, arelatively bright region occupies a large area in the area surrounded bythe solid line, while a relatively dark region occupies a large area inthe area surrounded by the dashed line.

That is, the areas surrounded by the solid lines are regions including arelatively large number of In atoms and the areas surrounded by thedashed lines are regions including a relatively small number of Inatoms. In FIG. 18C, the right portion of the area surrounded by thesolid line is relatively bright and the left portion thereof isrelatively dark. Thus, the area surrounded by the solid line is a regionincluding In_(X2)Zn_(Y2)O_(Z2), InO_(X1), or the like as a maincomponent.

The area surrounded by the solid line is a region including a relativelysmall number of Ga atoms and the area surrounded by the dashed line is aregion including a relatively large number of Ga atoms. In FIG. 18C, theupper left portion of the area surrounded by the dashed line isrelatively bright and the lower right portion thereof is relativelydark. Thus, the area surrounded by the dashed line is a region includingGaO_(X3), Ga_(X4)Zn_(Y4)O_(Z4), or the like as a main component.

Furthermore, as shown in FIGS. 18A to 18C, the In atoms are relativelymore uniformly distributed than the Ga atoms, and regions includingInO_(X1) as a main component are seemingly joined to each other througha region including In_(X2)Zn_(Y2)O_(Z2) as a main component. Thus, theregions including In_(X2)Zn_(Y2)O_(Z2) and InO_(X1) as main componentsextend like a cloud.

An In—Ga—Zn oxide having a composition in which the regions includingGaO_(X3) or the like as a main component and the regions includingIn_(X2)Zn_(Y2)O_(Z2) or InO_(X1) as a main component are unevenlydistributed and mixed can be referred to as a CAC-OS.

The crystal structure of the CAC-OS includes an nc structure. In anelectron diffraction pattern of the CAC-OS with the nc structure,several or more bright spots appear in addition to bright sports derivedfrom IGZO including a single crystal, a polycrystal, or a CAAC.Alternatively, the crystal structure is defined as having high luminanceregions appearing in a ring pattern in addition to the several or morebright spots.

As shown in FIGS. 18A to 18C, each of the regions including GaO_(X3) orthe like as a main component and the regions includingIn_(X2)Zn_(Y2)O_(Z2) or InO_(X1) as a main component has a size ofgreater than or equal to 0.5 nm and less than or equal to 10 nm, orgreater than or equal to 1 nm and less than or equal to 3 nm. Note thatit is preferable that a diameter of a region including each metalelement as a main component be greater than or equal to 1 nm and lessthan or equal to 2 nm in the EDX mapping images.

As described above, the CAC-OS has a structure different from that of anIGZO compound in which metal elements are evenly distributed, and hascharacteristics different from those of the IGZO compound. That is, inthe CAC-OS, regions including GaO_(X3) or the like as a main componentand regions including In_(X2)Zn_(Y2)O_(Z2) or InO_(X1) as a maincomponent are separated to form a mosaic pattern.

The conductivity of a region including In_(X2)Zn_(Y2)O_(Z2) or InO_(X1)as a main component is higher than that of a region including GaO_(X3)or the like as a main component. In other words, when carriers flowthrough regions including In_(X2)Zn_(Y2)O_(Z2) or InO_(X1) as a maincomponent, the conductivity of an oxide semiconductor is exhibited.Accordingly, when regions including In_(X2)Zn_(Y2)O_(Z2) or InO_(X1) asa main component are distributed in an oxide semiconductor like a cloud,high field-effect mobility (μ) can be achieved.

In contrast, the insulating property of a region including GaO_(X3) orthe like as a main component is higher than that of a region includingIn_(X2)Zn_(Y2)O_(Z2) or InO_(X1) as a main component. In other words,when regions including GaO_(X3) or the like as a main component aredistributed in an oxide semiconductor, leakage current can be suppressedand favorable switching operation can be achieved.

Accordingly, when a CAC-OS is used for a semiconductor element, theinsulating property derived from GaO_(X3) or the like and theconductivity derived from In_(X2)Zn_(Y2)O_(z2) or InO_(X1) complementeach other, whereby high on-state current (I_(on)) and high field-effectmobility (μ) can be achieved.

A semiconductor element including a CAC-OS has high reliability. Thus,the CAC-OS is suitably used in a variety of semiconductor devicestypified by a display.

Alternatively, silicon may be used as a semiconductor in which a channelof a transistor is formed. Silicon may be amorphous silicon but ispreferably silicon having crystallinity, such as microcrystallinesilicon, polycrystalline silicon, or single crystal silicon. Inparticular, polycrystalline silicon can be formed at a lower temperaturethan single crystal silicon and has higher field-effect mobility andhigher reliability than amorphous silicon.

The bottom-gate transistor described in this embodiment is preferablebecause the number of manufacturing steps can be reduced. When amorphoussilicon, which can be formed at a lower temperature than polycrystallinesilicon, is used for the semiconductor layer, materials with low heatresistance can be used for a wiring, an electrode, or a substrate belowthe semiconductor layer, resulting in wider choice of materials. Forexample, an extremely large glass substrate can be favorably used.Meanwhile, a top-gate transistor is preferable to form an impurityregion with ease in a self-aligned manner and reduce variation incharacteristics. The top-gate transistor is particularly preferable whenpolycrystalline silicon, single-crystal silicon, or the like isemployed.

[Conductive Layer]

Examples of materials for a gate, a source, and a drain of a transistorand a conductive layer such as a wiring or an electrode included in adisplay device include metals such as aluminum, titanium, chromium,nickel, copper, yttrium, zirconium, molybdenum, silver, tantalum, andtungsten, and an alloy containing any of these metals as its maincomponent. A single-layer structure or multi-layer structure including afilm containing any of these materials can be used. For example, thefollowing structures can be employed: a single-layer structure of analuminum film containing silicon, a two-layer structure in which analuminum film is stacked over a titanium film, a two-layer structure inwhich an aluminum film is stacked over a tungsten film, a two-layerstructure in which a copper film is stacked over acopper-magnesium-aluminum alloy film, a two-layer structure in which acopper film is stacked over a titanium film, a two-layer structure inwhich a copper film is stacked over a tungsten film, a three-layerstructure in which a titanium film or a titanium nitride film, analuminum film or a copper film, and a titanium film or a titaniumnitride film are stacked in this order, and a three-layer structure inwhich a molybdenum film or a molybdenum nitride film, an aluminum filmor a copper film, and a molybdenum film or a molybdenum nitride film arestacked in this order. Note that an oxide such as indium oxide, tinoxide, or zinc oxide may be used. Copper containing manganese ispreferably used because it increases shape controllability in etching.

Examples of a light-transmitting conductive material include aconductive oxide such as indium oxide, indium tin oxide, indium zincoxide, zinc oxide, or zinc oxide to which gallium is added, andgraphene. It is also possible to use, for example, a metal material suchas gold, silver, platinum, magnesium, nickel, tungsten, chromium,molybdenum, iron, cobalt, copper, palladium, or titanium; an alloymaterial containing any of these metal materials; or a nitride of any ofthese metal materials (e.g., titanium nitride). In the case of using themetal material or the alloy material (or the nitride thereof), the filmthickness is set small enough to transmit light. Alternatively, alayered film of any of the above materials can be used for theconductive layers. For example, a layered film of indium tin oxide andan alloy of silver and magnesium is preferably used because it canincrease the conductivity. These materials can also be used forconductive layers such as a variety of wirings and electrodes includedin a display device, and conductive layers (e.g., conductive layersserving as a pixel electrode or a common electrode) included in adisplay element.

[Insulating Layer]

Examples of an insulating material that can be used for the insulatinglayers include a resin such as an acrylic or epoxy resin, a resin havinga siloxane bond, and an inorganic insulating material such as siliconoxide, silicon oxynitride, silicon nitride oxide, silicon nitride, oraluminum oxide.

The light-emitting element is preferably provided between a pair ofinsulating films with low water permeability, in which case impuritiessuch as water can be prevented from entering the light-emitting element.Thus, a decrease in device reliability can be prevented.

Examples of the insulating film with low water permeability include afilm containing nitrogen and silicon (e.g., a silicon nitride film and asilicon nitride oxide film) and a film containing nitrogen and aluminum(e.g., an aluminum nitride film). Alternatively, a silicon oxide film, asilicon oxynitride film, an aluminum oxide film, or the like may beused.

For example, the moisture vapor transmission rate of the insulating filmwith low water permeability is lower than or equal to 1×10⁻⁵[g/(m²·day)], preferably lower than or equal to 1×10⁻⁶ [g/(m²·day)],further preferably lower than or equal to 1×10⁻⁷ [g/(m²·day)], stillfurther preferably lower than or equal to 1×10⁻⁸ [g/(m²·day)].

[Liquid Crystal Element]

The liquid crystal element can employ, for example, a vertical alignment(VA) mode. Examples of the vertical alignment mode include amulti-domain vertical alignment (MVA) mode, a patterned verticalalignment (PVA) mode, and an advanced super view (ASV) mode.

The liquid crystal element can employ any of a variety of modes. Forexample, the liquid crystal element can employ a twisted nematic (TN)mode, an in-plane switching (IPS) mode, a fringe field switching (FFS)mode, an axially symmetric aligned micro-cell (ASM) mode, an opticallycompensated birefringence (OCB) mode, a ferroelectric liquid crystal(FLC) mode, an antiferroelectric liquid crystal (AFLC) mode, or thelike, instead of employing a vertical alignment (VA) mode.

The liquid crystal element controls transmission or non-transmission oflight with the use of an optical modulation action of a liquid crystal.The optical modulation action of a liquid crystal is controlled by anelectric field applied to the liquid crystal (including a horizontalelectric field, a vertical electric field, and an oblique electricfield). Examples of the liquid crystal used for the liquid crystalelement include thermotropic liquid crystal, low-molecular liquidcrystal, high-molecular liquid crystal, polymer dispersed liquid crystal(PDLC), ferroelectric liquid crystal, and anti-ferroelectric liquidcrystal. Such a liquid crystal material exhibits a cholesteric phase, asmectic phase, a cubic phase, a chiral nematic phase, an isotropicphase, or the like depending on conditions.

As the liquid crystal material, a positive liquid crystal or a negativeliquid crystal may be used, and an appropriate liquid crystal materialis selected depending on the mode or design to be employed.

An alignment film can be provided to adjust the alignment of a liquidcrystal. In the case where a horizontal electric field mode is employed,a liquid crystal exhibiting a blue phase for which an alignment film isunnecessary may be used. A blue phase is one of liquid crystal phases.When the temperature of cholesteric liquid crystal is being increased,the blue phase is generated just before a cholesteric phase changes intoan isotropic phase. Since the blue phase appears only in a narrowtemperature range, a liquid crystal composition in which a chiralmaterial is mixed to account for several weight percent or more is usedfor the liquid crystal layer in order to improve the temperature range.The liquid crystal composition that includes a liquid crystal exhibitinga blue phase and a chiral material has a short response time and opticalisotropy; in addition, such a liquid crystal composition does notrequire the alignment process and has a small viewing angle dependence.An alignment film does not need to be provided and rubbing treatment isthus not necessary; accordingly, electrostatic discharge damage causedby the rubbing treatment can be prevented and defects and damage of theliquid crystal display device in the manufacturing process can bereduced.

The liquid crystal element may be a transmissive liquid crystal element,a reflective liquid crystal element, a semi-transmissive liquid crystalelement, or the like.

In one embodiment of the present invention, the reflective liquidcrystal element can be particularly used.

In the case where a transmissive or semi-transmissive liquid crystalelement is used, two polarizing plates are provided such that a pair ofsubstrates are sandwiched therebetween. Furthermore, a backlight isprovided on the outer side of the polarizing plate. As the backlight, adirect-below backlight or an edge-light backlight may be used. It ispreferable to use a direct-below backlight including an LED, with whichlocal dimming can be easily performed and contrast can be increased. Theedge-light backlight is preferably used to reduce the thickness of amodule including the backlight.

In the case where a reflective liquid crystal element is used, apolarizing plate is provided on the display surface side. In addition, alight diffusion plate is preferably provided on the display surface sideto improve visibility.

In the case where the reflective or the semi-transmissive liquid crystalelement is used, a front light may be provided outside the polarizingplate. As the front light, an edge-light front light is preferably used.A front light including an LED is preferably used to reduce powerconsumption.

[Light-Emitting Element] As the light-emitting element, a self-luminouselement can be used, and an element whose luminance is controlled bycurrent or voltage is included in the category of the light-emittingelement. For example, an LED, an organic EL element, an inorganic ELelement, or the like can be used.

The light-emitting element has a top-emission structure, abottom-emission structure, a dual-emission structure, or the like. Aconductive film that transmits visible light is used as the electrodethrough which light is extracted. A conductive film that reflectsvisible light is preferably used as the electrode through which light isnot extracted.

The EL layer includes at least a light-emitting layer. In addition tothe light-emitting layer, the EL layer may further include one or morelayers containing any of a substance with a high hole-injectionproperty, a substance with a high hole-transport property, ahole-blocking material, a substance with a high electron-transportproperty, a substance with a high electron-injection property, asubstance with a bipolar property (a substance with a high electron- andhole-transport property), and the like.

For the EL layer, either a low-molecular compound or a high-molecularcompound can be used, and an inorganic compound may also be used. Eachof the layers included in the EL layer can be formed by any of thefollowing methods: an evaporation method (including a vacuum evaporationmethod), a transfer method, a printing method, an inkjet method, acoating method, and the like.

When a voltage higher than the threshold voltage of the light-emittingelement is applied between a cathode and an anode, holes are injected tothe EL layer from the anode side and electrons are injected to the ELlayer from the cathode side. The injected electrons and holes arerecombined in the EL layer and a light-emitting substance contained inthe EL layer emits light.

In the case where a light-emitting element emitting white light is usedas the light-emitting element, the EL layer preferably contains two ormore kinds of light-emitting substances. For example, the two or morekinds of light-emitting substances are selected so as to emit light ofcomplementary colors to obtain white light emission. Specifically, it ispreferable to contain two or more selected from light-emittingsubstances emitting light of red (R), green (G), blue (B), yellow (Y),orange (O), and the like and light-emitting substances emitting lightcontaining two or more of spectral components of R, G, and B. Thelight-emitting element preferably emits light with a spectrum having twoor more peaks in the wavelength range of a visible light region (e.g.,350 nm to 750 nm). An emission spectrum of a material emitting lighthaving a peak in a yellow wavelength range preferably includes spectralcomponents also in green and red wavelength ranges.

A light-emitting layer containing a light-emitting material emittinglight of one color and a light-emitting layer containing alight-emitting material emitting light of another color are preferablystacked in the EL layer. For example, a plurality of light-emittinglayers in the EL layer may be stacked in contact with each other or maybe stacked with a region not including any light-emitting materialtherebetween. For example, between a fluorescent layer and aphosphorescent layer, a region containing no light-emitting element butthe same material as one in the fluorescent layer or phosphorescentlayer (for example, a host material or an assist material) may beprovided. This facilitates the manufacture of the light-emitting elementand reduces the drive voltage.

The light-emitting element may be a single element including one ELlayer or a tandem element in which a plurality of EL layers are stackedwith a charge generation layer therebetween.

The conductive film that transmits visible light can be formed using,for example, indium oxide, indium tin oxide, indium zinc oxide, zincoxide, or zinc oxide to which gallium is added. Alternatively, a film ofa metal material such as gold, silver, platinum, magnesium, nickel,tungsten, chromium, molybdenum, iron, cobalt, copper, palladium, ortitanium; an alloy containing any of these metal materials; or a nitrideof any of these metal materials (e.g., titanium nitride) that is thinenough to have a light-transmitting property can be used. Alternatively,a layered film of any of the above materials can be used for theconductive layer. For example, a layered film of indium tin oxide and analloy of silver and magnesium is preferably used, in which caseconductivity can be increased. Further alternatively, graphene or thelike may be used.

For the conductive film that reflects visible light, for example, ametal material such as aluminum, gold, platinum, silver, nickel,tungsten, chromium, molybdenum, iron, cobalt, copper, or palladium or analloy containing any of these metal materials can be used. Lanthanum,neodymium, germanium, or the like may be added to the metal material orthe alloy. Alternatively, an alloy containing aluminum (an aluminumalloy) such as an alloy of aluminum and titanium, an alloy of aluminumand nickel, or an alloy of aluminum and neodymium may be used.Alternatively, an alloy containing silver such as an alloy of silver andcopper, an alloy of silver and palladium, or an alloy of silver andmagnesium may be used. An alloy containing silver and copper ispreferable because of its high heat resistance. Furthermore, when ametal film or a metal oxide film is stacked in contact with an aluminumfilm or an aluminum alloy film, oxidation can be suppressed. Examples ofa material for the metal film or the metal oxide film include titaniumand titanium oxide. Alternatively, the above conductive film thattransmits visible light and a film containing a metal material may bestacked. For example, a stack of indium tin oxide and silver, a stack ofindium tin oxide and an alloy of silver and magnesium, or the like canbe used.

Each of the electrodes can be formed by an evaporation method or asputtering method. Alternatively, a discharging method such as an inkjetmethod, a printing method such as a screen printing method, or a platingmethod may be used.

Note that the aforementioned light-emitting layer and layers containinga substance with a high hole-injection property, a substance with a highhole-transport property, a substance with a high electron-transportproperty, a substance with a high electron-injection property, and asubstance with a bipolar property may include an inorganic compound suchas a quantum dot or a high molecular compound (e.g., an oligomer, adendrimer, and a polymer). For example, a quantum dot included in thelight-emitting layer can serve as a light-emitting material.

The quantum dot may be a colloidal quantum dot, an alloyed quantum dot,a core-shell quantum dot, a core quantum dot, or the like. The quantumdot containing elements belonging to Groups 12 and 16, elementsbelonging to Groups 13 and 15, or elements belonging to Groups 14 and16, may be used. Alternatively, the quantum dot containing an elementsuch as cadmium, selenium, zinc, sulfur, phosphorus, indium, tellurium,lead, gallium, arsenic, or aluminum may be used.

[Adhesive Layer]

As the adhesive layer, a variety of curable adhesives, e.g., aphotocurable adhesive such as an ultraviolet curable adhesive, areactive curable adhesive, a thermosetting adhesive, and an anaerobicadhesive can be used. Examples of these adhesives include an epoxyresin, an acrylic resin, a silicone resin, a phenol resin, a polyimideresin, an imide resin, a polyvinyl chloride (PVC) resin, a polyvinylbutyral (PVB) resin, and an ethylene vinyl acetate (EVA) resin. Inparticular, a material with low moisture permeability, such as an epoxyresin, is preferred. Alternatively, a two-component resin may be used.Still alternatively, an adhesive sheet or the like may be used.

Furthermore, the above-mentioned resin may include a drying agent. Forexample, a substance that adsorbs moisture by chemical adsorption, suchas oxide of an alkaline earth metal (e.g., calcium oxide or bariumoxide), can be used. Alternatively, a substance that adsorbs moisture byphysical adsorption, such as zeolite or silica gel, may be used. Thedrying agent is preferably included because it can prevent impuritiessuch as moisture from entering the element and improves the reliabilityof the display panel.

In addition, when the resin is mixed with a filler with a highrefractive index or a light-scattering member, light extractionefficiency can be enhanced. For example, titanium oxide, barium oxide,zeolite, zirconium, or the like can be used.

[Connection Layer]

As the connection layers, an anisotropic conductive film (ACF), ananisotropic conductive paste (ACP), or the like can be used.

[Coloring Layer]

Examples of a material that can be used for the coloring layers includea metal material, a resin material, and a resin material containing apigment or dye.

[Light-Blocking Layer]

Examples of a material that can be used for the light-blocking layerinclude carbon black, titanium black, a metal, a metal oxide, and acomposite oxide containing a solid solution of a plurality of metaloxides. The light-blocking layer may be a film containing a resinmaterial or a thin film of an inorganic material such as a metal. Alayered film containing the material of the coloring layer can also beused for the light-blocking layer. For example, a stacked-layerstructure of a film containing a material of a coloring layer thattransmits light of a certain color and a film containing a material of acoloring layer that transmits light of another color can be employed.The coloring layer and the light-blocking layer are preferably formedusing the same material, in which case the same manufacturing apparatuscan be used and the process can be simplified.

At least part of this embodiment can be implemented in combination withany of the other embodiments and the other examples described in thisspecification as appropriate.

Embodiment 4

In this embodiment, an example of a transistor that can be used as thetransistors described in the above embodiments will be described withreference to drawings.

The display device of one embodiment of the present invention can befabricated by using a transistor with any of various modes, such as abottom-gate transistor or a top-gate transistor. Therefore, a materialfor a semiconductor layer or the structure of a transistor can be easilychanged depending on the existing production line.

<5-1. Bottom-Gate Transistor>

FIG. 19A1 is a cross-sectional view in the channel length direction of atransistor 810 that is a channel-protective transistor, which is a typeof bottom-gate transistor. FIGS. 19A2, 19B1, 19B2, 19C1, and 19C2 areeach a cross-sectional view of a bottom-gate transistor in the channellength direction.

In FIG. 19A1, the transistor 810 is over a substrate 771. The transistor810 includes an electrode 746 over the substrate 771 with an insulatinglayer 772 therebetween. The transistor 810 includes a semiconductorlayer 742 over the electrode 746 with an insulating layer 726therebetween. The electrode 746 can function as a gate electrode. Theinsulating layer 726 can function as a gate insulating layer.

The transistor 810 includes an insulating layer 741 over a channelformation region in the semiconductor layer 742. The transistor 810includes an electrode 744 a and an electrode 744 b that are in contactwith part of the semiconductor layer 742 and over the insulating layer726. The electrode 744 a can function as one of a source electrode and adrain electrode. The electrode 744 b can function as the other of thesource electrode and the drain electrode. Part of the electrode 744 aand part of the electrode 744 b are formed over the insulating layer741.

The insulating layer 741 can function as a channel protective layer.With the insulating layer 741 over the channel formation region, thesemiconductor layer 742 can be prevented from being exposed at the timeof forming the electrodes 744 a and 744 b. Thus, the channel formationregion in the semiconductor layer 742 can be prevented from being etchedat the time of forming the electrodes 744 a and 744 b. With oneembodiment of the present invention, a transistor with favorableelectrical characteristics can be provided.

The transistor 810 includes an insulating layer 728 over the electrode744 a, the electrode 744 b, and the insulating layer 741 and furtherincludes an insulating layer 729 over the insulating layer 728.

In the case where an oxide semiconductor is used for the semiconductorlayer 742, a material capable of removing oxygen from part of thesemiconductor layer 742 to generate oxygen vacancies is preferably usedfor regions of the electrodes 744 a and 744 b that are in contact withat least the semiconductor layer 742. The carrier concentration in theregions of the semiconductor layer 742 where oxygen vacancies aregenerated is increased, so that the regions become n-type regions (n⁺layers). Accordingly, the regions can function as a source region and adrain region. When an oxide semiconductor is used for the semiconductorlayer 742, examples of the material capable of removing oxygen from thesemiconductor layer 742 to generate oxygen vacancies include tungstenand titanium.

Formation of the source region and the drain region in the semiconductorlayer 742 makes it possible to reduce contact resistance between thesemiconductor layer 742 and each of the electrodes 744 a and 744 b.Accordingly, the electric characteristics of the transistor, such as thefield-effect mobility and the threshold voltage, can be favorable.

In the case where a semiconductor such as silicon is used for thesemiconductor layer 742, a layer that functions as an n-typesemiconductor or a p-type semiconductor is preferably present betweenthe semiconductor layer 742 and the electrode 744 a and between thesemiconductor layer 742 and the electrode 744 b. The layer thatfunctions as an n-type semiconductor or a p-type semiconductor canfunction as the source region or the drain region in the transistor.

The insulating layer 729 is preferably formed using a material that canprevent or reduce diffusion of impurities into the transistor from theoutside. The formation of the insulating layer 729 may be omitted whereappropriate.

A transistor 811 illustrated in FIG. 19A2 is different from thetransistor 810 in that an electrode 723 that can serve as a back gateelectrode is provided over the insulating layer 729. The electrode 723can be formed using a material and a method similar to those of theelectrode 746.

In general, the back gate electrode is formed using a conductive layerand positioned so that the channel formation region of the semiconductorlayer is between the gate electrode and the back gate electrode. Thus,the back gate electrode can function in a manner similar to that of thegate electrode. The potential of the back gate electrode may be the sameas that of the gate electrode or may be a ground (GND) potential or apredetermined potential. By changing the potential of the back gateelectrode independently of the potential of the gate electrode, thethreshold voltage of the transistor can be changed.

FIG. 20A1 illustrates a cross-sectional view in the channel widthdirection of the transistor 810. FIG. 20A2 illustrates a cross-sectionalview in the channel width direction of the transistor 811. FIG. 20B1illustrates a cross-sectional view in the channel width direction of atransistor 820. FIG. 20B2 illustrates a cross-sectional view in thechannel width direction of a transistor 821. FIG. 20C1 illustrates across-sectional view in the channel width direction of a transistor 825.FIG. 20C2 illustrates a cross-sectional view in the channel widthdirection of a transistor 826.

In each of the structures illustrated in FIG. 20B2 and FIG. 20C2, thegate electrode is connected to the back gate electrode, and the gateelectrode and the back gate electrode have the same potential.

Furthermore, in each of the structures illustrated in FIG. 20B2 and FIG.20C2, the semiconductor layer 742 is positioned between the gateelectrode and the back gate electrode. The length in the channel widthdirection of each of the gate electrode and the back gate electrode isgreater than the length in the channel width direction of thesemiconductor layer 742. In the channel width direction, the whole ofthe semiconductor layer 742 is covered with the gate electrode or theback gate electrode with the insulating layers 726, 741, 728, and 729positioned therebetween. With the structure, the semiconductor layer 742included in the transistor can be surrounded by the electric field ofthe gate electrode and the back gate electrode.

A device structure of a transistor, like that of the transistor 821 orthe transistor 826, in which the electric field of a gate electrode anda back gate electrode electrically surrounds the semiconductor layer 742in which a channel region is formed can be referred to as a surroundedchannel (S-channel) structure.

With the S-channel structure, an electric field for inducing a channelcan be effectively applied to the semiconductor layer 742 by one or bothof the gate electrode and the back gate electrode, which enablesimprovement in the current drive capability of the transistor and offershigh on-state current characteristics. Since the on-state current can beincreased, the transistor can be minimized. Furthermore, with theS-channel structure, the mechanical strength of the transistor can beincreased.

The electrode 746 and the electrode 723 can each serve as a gateelectrode. Thus, the insulating layers 726, 728, and 729 can each serveas a gate insulating layer. The electrode 723 may be between theinsulating layers 728 and 729.

In the case where one of the electrode 746 and the electrode 723 issimply referred to as a “gate electrode”, the other can be referred toas a “back gate electrode”. For example, in the transistor 811, in thecase where the electrode 723 is referred to as a “gate electrode”, theelectrode 746 is referred to as a “back gate electrode”. In the casewhere the electrode 723 is used as a “gate electrode”, the transistor811 can be regarded as a kind of top-gate transistor. Alternatively, oneof the electrode 746 and the electrode 723 may be referred to as a“first gate electrode”, and the other may be referred to as a “secondgate electrode”.

By providing the electrode 746 and the electrode 723 with thesemiconductor layer 742 therebetween and setting the potentials of theelectrode 746 and the electrode 723 to be the same, a region of thesemiconductor layer 742 through which carriers flow is enlarged in thefilm thickness direction; thus, the number of transferred carriers isincreased. As a result, the on-state current and field-effect mobilityof the transistor 811 are increased.

Therefore, the transistor 811 has a high on-state current for its area.That is, the area of the transistor 811 can be small for a requiredon-state current. With one embodiment of the present invention, the areaoccupied by a transistor can be reduced. Therefore, with one embodimentof the present invention, a highly integrated semiconductor device canbe provided.

Furthermore, the gate electrode and the back gate electrode are formedusing conductive layers and thus each have a function of preventing anelectric field generated outside the transistor from influencing thesemiconductor layer in which the channel is formed (in particular, anelectric field blocking function against static electricity and thelike). When the back gate electrode is formed larger than thesemiconductor layer such that the semiconductor layer is covered withthe back gate electrode, the electric field blocking function can beenhanced.

When the back gate electrode is formed using a light-blocking conductivefilm, light can be prevented from entering the semiconductor layer fromthe back gate electrode side. Therefore, photodegradation of thesemiconductor layer can be prevented and deterioration in electricalcharacteristics of the transistor, such as a shift of the thresholdvoltage, can be prevented.

With one embodiment of the present invention, a transistor with highreliability can be provided. Moreover, a semiconductor device with highreliability can be provided.

FIG. 19B1 is a cross-sectional view of the transistor 820 that is achannel-protective transistor, which is a type of bottom-gatetransistor. The transistor 820 has substantially the same structure asthe transistor 810 but is different from the transistor 810 in that theinsulating layer 741 covers an end portion of the semiconductor layer742. The semiconductor layer 742 is electrically connected to theelectrode 744 a in an opening formed by selectively removing part of theinsulating layer 741 that overlaps with the semiconductor layer 742. Thesemiconductor layer 742 is electrically connected to the electrode 744 bin another opening formed by selectively removing part of the insulatinglayer 741 that overlaps with the semiconductor layer 742. A region ofthe insulating layer 741 that overlaps with the channel formation regioncan function as a channel protective layer.

The transistor 821 illustrated in FIG. 19B2 is different from thetransistor 820 in that the electrode 723 that can serve as a back gateelectrode is positioned over the insulating layer 729.

With the insulating layer 741, the semiconductor layer 742 can beprevented from being exposed at the time of forming the electrodes 744 aand 744 b. Thus, the semiconductor layer 742 can be prevented from beingreduced in thickness at the time of forming the electrodes 744 a and 744b.

The distance between the electrode 744 a and the electrode 746 and thedistance between the electrode 744 b and the electrode 746 in thetransistors 820 and 821 are larger than those in the transistors 810 and811. Thus, the parasitic capacitance generated between the electrode 744a and the electrode 746 can be reduced. Moreover, the parasiticcapacitance generated between the electrode 744 b and the electrode 746can be reduced. According to one embodiment of the present invention, atransistor with excellent electrical characteristics can be provided.

The transistor 825 illustrated in FIG. 19C1 is a channel-etchedtransistor that is a type of bottom-gate transistor. In the transistor825, the electrodes 744 a and 744 b are formed without providing theinsulating layer 741. Thus, part of the semiconductor layer 742 that isexposed at the time of forming the electrodes 744 a and 744 b is etchedin some cases. However, since the insulating layer 741 is not provided,the productivity of the transistor can be increased.

The transistor 826 illustrated in FIG. 19C2 is different from thetransistor 825 in that the electrode 723 that can serve as a back gateelectrode is positioned over the insulating layer 729.

<5-2. Top-Gate Transistor>

FIG. 21A1 is a cross-sectional view in the channel length direction of atransistor 830 that is a type of top-gate transistor. The transistor 830includes the semiconductor layer 742 over the insulating layer 772, theelectrodes 744 a and 744 b that are over the semiconductor layer 742 andthe insulating layer 772 and in contact with parts of the semiconductorlayer 742, the insulating layer 726 over the semiconductor layer 742 andthe electrodes 744 a and 744 b, and the electrode 746 over theinsulating layer 726. FIGS. 21A2, 21A3, 21B1, and 21B2 are each across-sectional view of a top-gate transistor in the channel lengthdirection.

Since the electrode 746 overlaps with neither the electrode 744 a northe electrode 744 b in the transistor 830, the parasitic capacitancegenerated between the electrodes 746 and 744 a and the parasiticcapacitance generated between the electrodes 746 and 744 b can bereduced. After the formation of the electrode 746, an impurity 755 isintroduced into the semiconductor layer 742 using the electrode 746 as amask, so that an impurity region can be formed in the semiconductorlayer 742 in a self-aligned manner (see FIG. 21A3). According to oneembodiment of the present invention, a transistor with favorableelectrical characteristics can be provided.

The introduction of the impurity 755 can be performed with an ionimplantation apparatus, an ion doping apparatus, or a plasma treatmentapparatus.

As the impurity 755, for example, at least one kind of element of Group13 elements and Group 15 elements can be used. In the case where anoxide semiconductor is used for the semiconductor layer 742, it ispossible to use at least one kind of element of a rare gas, hydrogen,and nitrogen as the impurity 755.

A transistor 831 illustrated in FIG. 21A2 is different from thetransistor 830 in that the electrode 723 and the insulating layer 727are included. The transistor 831 includes the electrode 723 over theinsulating layer 772 and the insulating layer 727 over the electrode723. The electrode 723 can function as a back gate electrode. Thus, theinsulating layer 727 can function as a gate insulating layer. Theinsulating layer 727 can be formed using a material and a method similarto those of the insulating layer 726.

Like the transistor 811, the transistor 831 has a high on-state currentfor its area. That is, the area of the transistor 831 can be small for arequired on-state current. With one embodiment of the present invention,the area occupied by a transistor can be reduced. Therefore, with oneembodiment of the present invention, a highly integrated semiconductordevice can be provided.

A transistor 840 illustrated in FIG. 21B1 is a type of top-gatetransistor. The transistor 840 is different from the transistor 830 inthat the semiconductor layer 742 is formed after the formation of theelectrodes 744 a and 744 b. A transistor 841 illustrated in FIG. 21B2 isdifferent from the transistor 840 in that the electrode 723 and theinsulating layer 727 are included. In the transistors 840 and 841, partof the semiconductor layer 742 is formed over the electrode 744 a andanother part of the semiconductor layer 742 is formed over the electrode744 b.

Like the transistor 811, the transistor 841 has a high on-state currentfor its area. That is, the area of the transistor 841 can be small for arequired on-state current. With one embodiment of the present invention,the area occupied by a transistor can be reduced. Therefore, with oneembodiment of the present invention, a highly integrated semiconductordevice can be provided.

FIG. 22A1 illustrates a cross-sectional view in the channel widthdirection of the transistor 830 in FIG. 21A1. FIG. 22A2 illustrates across-sectional view in the channel width direction of the transistor831 in FIG. 21A2. FIG. 22B1 illustrates a cross-sectional view in thechannel width direction of the transistor 840 in FIG. 21B1. FIG. 22B2illustrates a cross-sectional view in the channel width direction of thetransistor 841 in FIG. 21B2.

Note that the transistors 831 and 841 each have the above-describedS-channel structure; however, one embodiment of the present invention isnot limited to this, and the transistors 831 and 841 do not necessarilyhave the S-channel structure.

FIGS. 23A1, 23A2, 23A3, 23B1, 23B2, 23C1, and 23C2 and FIGS. 24A1, 24A2,24B1, 24B2, 24C1, and 24C2 show top-gate transistors different fromthose illustrated in FIGS. 21A1, 21A2, 21A3, 21B1, and 21B2 and FIGS.22A1, 22A2, 22B1, and 22B2.

FIG. 23A1 is a cross-sectional view in the channel length direction of atransistor 842. FIG. 23A2 is a cross-sectional view in the channellength direction of a transistor 843. FIG. 23B1 is a cross-sectionalview in the channel length direction of a transistor 844. FIG. 23B2 is across-sectional view in the channel length direction of a transistor845. FIG. 23C1 is a cross-sectional view in the channel length directionof a transistor 846. FIG. 23C2 is a cross-sectional view in the channellength direction of a transistor 847.

FIG. 23A3 is a cross-sectional view in the channel length direction,which illustrates a step of manufacturing the transistor 842.

The transistor 842 illustrated in FIG. 23A1 is different from thetransistor 830 or 840 in that the electrodes 744 a and 744 b are formedafter the formation of the insulating layer 729. The electrodes 744 aand 744 b are electrically connected to the semiconductor layer 742 inopenings formed in the insulating layers 728 and 729.

Part of the insulating layer 726 that does not overlap with theelectrode 746 is removed, and the impurity 755 is introduced into thesemiconductor layer 742 using the electrode 746 and the remaininginsulating layer 726 as a mask, so that an impurity region can be formedin the semiconductor layer 742 in a self-aligned manner (see FIG. 23A3).The transistor 842 includes a region where the insulating layer 726extends beyond an end portion of the electrode 746. The semiconductorlayer 742 in a region into which the impurity 755 is introduced throughthe insulating layer 726 has a lower impurity concentration than thesemiconductor layer 742 in a region into which the impurity 755 isintroduced without through the insulating layer 726. Thus, a lightlydoped drain (LDD) region is formed in a region of the semiconductorlayer 742 that does not overlap with the electrode 746.

The transistor 843 illustrated in FIG. 23A2 is different from thetransistor 842 in that the electrode 723 is included. The transistor 843includes the electrode 723 that is over the substrate 771 and overlapswith the semiconductor layer 742 with the insulating layer 772therebetween. The electrode 723 can function as a back gate electrode.

As in the transistor 844 illustrated in FIG. 23B1 and the transistor 845illustrated in FIG. 23B2, the insulating layer 726 in a region that doesnot overlap with the electrode 746 may be removed. Alternatively, as inthe transistor 846 illustrated in FIG. 23C1 and the transistor 847illustrated in FIG. 23C2, the insulating layer 726 may be left.

In the transistors 842 to 847, after the formation of the electrode 746,the impurity 755 is introduced into the semiconductor layer 742 usingthe electrode 746 as a mask, so that an impurity region can be formed inthe semiconductor layer 742 in a self-aligned manner. With oneembodiment of the present invention, a transistor with favorableelectrical characteristics can be provided. With one embodiment of thepresent invention, a highly integrated semiconductor device can beprovided.

FIG. 24A1 illustrates a cross-sectional view in the channel widthdirection of the transistor 842 in FIG. 23A1. FIG. 24A2 illustrates across-sectional view in the channel width direction of the transistor843 in FIG. 23A2. FIG. 24B1 illustrates a cross-sectional view in thechannel width direction of the transistor 844 in FIG. 23B1. FIG. 24B2illustrates a cross-sectional view in the channel width direction of thetransistor 845 in FIG. 23B2. FIG. 24C1 illustrates a cross-sectionalview in the channel width direction of the transistor 846 in FIG. 23C1.FIG. 24C2 illustrates a cross-sectional view in the channel widthdirection of the transistor 847 in FIG. 23C2.

Note that the transistors 843, 845, and 847 each have theabove-described S-channel structure; however, one embodiment of thepresent invention is not limited to this, and the transistors 843, 845,and 847 do not necessarily have the S-channel structure.

FIGS. 25A1 and 25A2 illustrate a modification example of the transistor845 illustrated in FIG. 23B2 and FIG. 24B2. FIG. 25A1 is across-sectional view in the channel length direction of a transistor845A, and FIG. 25A2 is a cross-sectional view in the channel widthdirection of the transistor 845A.

The transistor 845A in FIGS. 25A1 and 25A2 is the same as the transistor845 except the positions of the insulating layers 729 and 728.

At least part of this embodiment can be implemented in combination withany of the other embodiments and the other examples described in thisspecification as appropriate.

Embodiment 5

In this embodiment, a display module using the display device of oneembodiment of the present invention will be described. In a displaymodule 8000 in FIG. 26A, a display panel 8006 connected to an FPC 8005,a frame 8009, a printed circuit board 8010, and a battery 8011 arebetween an upper cover 8001 and a lower cover 8002.

The display device of one embodiment of the present invention can beused for the display panel 8006, for example. In that case, a displaymodule that offers high visibility and low power consumption can beobtained. The shapes and sizes of the upper cover 8001 and the lowercover 8002 can be changed as appropriate in accordance with the size ofthe display panel 8006.

In addition, the display panel 8006 includes the touch sensor of oneembodiment of the present invention.

The frame 8009 protects the display panel 8006 and functions as anelectromagnetic shield for blocking electromagnetic waves generated bythe operation of the printed circuit board 8010. The frame 8009 can alsofunction as a radiator plate.

The printed circuit board 8010 has a power supply circuit and a signalprocessing circuit for outputting a video signal and a clock signal. Asa power source for supplying power to the power supply circuit, anexternal commercial power source or the battery 8011 provided separatelymay be used. The battery 8011 can be omitted in the case of using acommercial power source.

FIG. 26B is a cross-sectional schematic view of the display module 8000with an optical touch sensor.

The display module 8000 includes a light-emitting portion 8015 and alight-receiving portion 8016 that are provided on the printed circuitboard 8010. A pair of light guide portions (a light guide portion 8017 aand a light guide portion 8017 b) is provided in a region surrounded bythe upper cover 8001 and the lower cover 8002.

The display panel 8006 overlaps with the printed circuit board 8010 andthe battery 8011 with the frame 8009 located therebetween. The displaypanel 8006 and the frame 8009 are fixed to the light guide portion 8017a and the light guide portion 8017 b.

Light 8018 emitted from the light-emitting portion 8015 travels over thedisplay panel 8006 through the light guide portion 8017 a and reachesthe light-receiving portion 8016 through the light guide portion 8017 b.For example, blocking of the light 8018 by a sensing target such as afinger or a stylus can be detected as touch operation.

A plurality of light-emitting portions 8015 are provided along twoadjacent sides of the display panel 8006, for example. A plurality oflight-receiving portions 8016 are provided so as to face thelight-emitting portions 8015 with the display panel 8006 locatedtherebetween. Accordingly, information about the position of touchoperation can be obtained.

As the light-emitting portion 8015, a light source such as an LEDelement can be used. It is particularly preferable to use a light sourcethat emits infrared light, which is not visually recognized by users andis harmless to users, as the light-emitting portion 8015.

As the light-receiving portion 8016, a photoelectric element thatreceives light emitted by the light-emitting portion 8015 and convertsit into an electrical signal can be used. A photodiode that can receiveinfrared light can be favorably used.

For the light guide portions 8017 a and 8017 b, members that transmit atleast the light 8018 can be used. With the use of the light guideportions 8017 a and 8017 b, the light-emitting portion 8015 and thelight-receiving portion 8016 can be placed under the display panel 8006,and a malfunction of the touch sensor due to external light reaching thelight-receiving portion 8016 can be suppressed. It is particularlypreferable to use a resin that absorbs visible light and transmitsinfrared light. This is more effective in suppressing the malfunction ofthe touch sensor.

At least part of this embodiment can be implemented in combination withany of the other embodiments and the other examples described in thisspecification as appropriate.

Embodiment 6

Examples of an electronic device that can use the display device of oneembodiment of the present invention include display devices, personalcomputers, image storage devices or image reproducing devices providedwith storage media, cellular phones, game machines (including portablegame machines), portable data terminals, e-book readers, cameras such asvideo cameras and digital still cameras, goggle-type displays (headmounted displays), navigation systems, audio reproducing devices (e.g.,car audio players and digital audio players), copiers, facsimiles,printers, multifunction printers, automated teller machines (ATM), andvending machines. FIGS. 27A to 27F illustrate specific examples of theseelectronic devices.

FIG. 27A is a television that includes a housing 971, a display portion973, an operation key 974, speakers 975, communication connectionterminal 976, an optical sensor 977, and the like. The display portion973 is provided with a touch sensor, with which an input operation canbe performed. The display portion 973 including the display device ofone embodiment of the present invention can have reduced powerconsumption.

FIG. 27B illustrates a portable game machine that includes a housing901, a display portion 903, a microphone 905, speakers 906, operationkeys 907, a camera 909, and the like. The display portion 903 includingthe display device of one embodiment of the present invention can havereduced power consumption.

FIG. 27C illustrates a digital camera that includes a housing 961, ashutter button 962, a microphone 963, a speaker 967, a display portion965, operation keys 966, and the like. The display portion 965 includingthe display device of one embodiment of the present invention can havereduced power consumption.

FIG. 27D illustrates a wristwatch information terminal that includes ahousing 931, a display portion 932, a wristband 933, operation buttons935, a winder 936, a camera 939, and the like. The display portion 932may be a touch panel. The display portion 932 including the displaydevice of one embodiment of the present invention can have reduced powerconsumption.

FIG. 27E is an example of a mobile phone that includes a housing 951, adisplay portion 952, an operation button 953, an external connectionport 954, a speaker 955, a microphone 956, a camera 957, and the like.The mobile phone includes a touch sensor in the display portion 952.Operations such as making a call and inputting a character can beperformed by touch on the display portion 952 with a finger, a stylus,or the like. The display portion 952 including the display device of oneembodiment of the present invention can have reduced power consumption.

FIG. 27F illustrates a portable data terminal that includes a housing911, a display portion 912, a camera 919, and the like. The touch panelfunction of the display portion 912 enables input and output ofinformation. The display portion 912 including the display device of oneembodiment of the present invention can have reduced power consumption.

FIGS. 28A and 28B illustrate foldable electronic devices.

An electronic device 920 shown in FIG. 28A includes a housing 921 a, ahousing 921 b, a hinge 923, a display portion 922 a, a display portion922 b, and the like. The display portion 922 a and the display portion922 b are incorporated in the housing 921 a and the housing 921 b,respectively.

The housing 921 a and the housing 921 b are rotatably joined to eachother by the hinge 923. The electronic device 920 can be changed inshape between a state where the housing 921 a and the housing 921 b areclosed and a state where the housing 921 a and the housing 921 b areopened as illustrated in FIG. 28A. Thus, the electronic device 920 hashigh portability when carried and excellent visibility when used becauseof its large display region.

The hinge 923 preferably includes a locking mechanism so that an angleformed between the housing 921 a and the housing 921 b does not becomelarger than a predetermined angle when the housing 921 a and the housing921 b are opened. For example, an angle at which they become locked(they are not opened any further) is preferably greater than or equal to90° and less than 180° and can be typically 90°, 120°, 135°, 150°, orthe like. In that case, the convenience, the safety, and the reliabilitycan be improved.

At least one of the display portion 922 a and the display portion 922 bcan function as a touch panel and be controlled with a finger, a stylus,or the like.

One of the housing 921 a and the housing 921 b is provided with awireless communication module, and data can be transmitted and receivedthrough a computer network such as the Internet, a local area network(LAN), or Wi-Fi (registered trademark).

One flexible display may be incorporated in the display portion 922 aand the display portion 922 b. In that case, an image can be displayedcontinuously across the display portion 922 a and the display portion922 b.

In an electronic device 980 illustrated in FIG. 28B, a flexible displayportion 982 is across a housing 981 a and a housing 981 b which arejoined to each other by a hinge 983.

At least part of the display portion 982 can be bent. The displayportion 982 can display an image while being bent since pixels arecontinuously arranged from the housing 981 a to the housing 981 b.

The hinge 983 includes the above-described locking mechanism, and thusexcessive force is not applied to the display portion 982; therefore,breakage of the display portion 982 can be prevented. Consequently, ahighly reliable electronic device can be obtained.

At least part of this embodiment can be implemented in combination withany of the other embodiments and the other examples described in thisspecification as appropriate.

This application is based on Japanese Patent Application Serial No.2016-182655 filed with Japan Patent Office on Sep. 20, 2016, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A display device comprising: a first substrate; asecond substrate provided over the first substrate; a first elementlayer including a first display element, the first element layerprovided over the first substrate and below the second substrate; asecond element layer including a first transistor and a secondtransistor, the second element layer provided over the first substrateand below the first element layer; a third element layer including asecond display element, the third element layer provided over the firstsubstrate and below the second element layer; an input device providedover the first element layer and below the second substrate; and adriver circuit provided over the first substrate, wherein the firstsubstrate and the second substrate overlap with each other, wherein thefirst display element, the second display element, the first transistor,and the second transistor are between a first surface of the firstsubstrate and a first surface of the second substrate, wherein the firstdisplay element is a reflective liquid crystal element electricallyconnected to the first transistor, wherein the second display element isa light-emitting element electrically connected to the secondtransistor, wherein a second surface of the second substrate opposite tothe first surface of the second substrate is provided with a firstanti-reflection layer, wherein the first surface of the first substrateis provided with the driver circuit, and wherein the input device andthe driver circuit are electrically connected to each other through anFPC.
 2. The display device according to claim 1, wherein the firstdisplay element, the second display element, the first transistor, andthe second transistor are in the same pixel unit.
 3. The display deviceaccording to claim 1, wherein the driver circuit is configured to drivethe first display element, the second display element, and the inputdevice.
 4. The display device according to claim 1, further comprising asecond anti-reflection layer provided on the first surface of the secondsubstrate.
 5. The display device according to claim 1, wherein the firstanti-reflection layer is a dielectric layer.
 6. The display deviceaccording to claim 1, wherein the first anti-reflection layer has ananti-glare pattern.
 7. The display device according to claim 1, whereinthe input device includes a wiring including a first layer provided onthe first surface of the second substrate and a second layer in contactwith the first layer, and wherein the first layer has a lowerreflectance than the second layer.
 8. The display device according toclaim 1, further comprising a light diffusion plate and a polarizingplate under the input device and over the first element layer.
 9. Thedisplay device according to claim 1, wherein each of the firsttransistor and the second transistor has a channel formation regioncomprising a metal oxide.
 10. An electronic device comprising: thedisplay device according to claim 1; a first housing; a second housing;and a hinge, wherein the display device is across the first housing andthe second housing that are joined to each other by the hinge, andwherein part of the display device is capable of being bent.
 11. Adisplay device comprising: a first substrate; a first element layerincluding a first display element, the first element layer provided overthe first substrate; a second element layer including a first transistorand a second transistor, the second element layer provided over thefirst substrate and below the first element layer; a third element layerincluding a second display element, the third element layer providedover the first substrate and below the second element layer; a drivercircuit over the first substrate; an input device over the first elementlayer; a second substrate over the input device; and a firstanti-reflection layer over the second substrate, wherein the firstdisplay element is a reflective liquid crystal element electricallyconnected to the first transistor, wherein the second display element isa light-emitting element electrically connected to the secondtransistor, and wherein the input device and the driver circuit areelectrically connected to each other through an FPC.
 12. The displaydevice according to claim 11, wherein the first display element, thesecond display element, the first transistor, and the second transistorare in the same pixel unit.
 13. The display device according to claim11, wherein the driver circuit is configured to drive the first displayelement, the second display element, and the input device.
 14. Thedisplay device according to claim 11, further comprising a secondanti-reflection layer between the second substrate and the input device.15. The display device according to claim 11, wherein the firstanti-reflection layer is a dielectric layer.
 16. The display deviceaccording to claim 11, wherein the first anti-reflection layer has ananti-glare pattern.
 17. The display device according to claim 11,wherein the input device includes a wiring including a first layeradjacent to the second substrate and a second layer in contact with thefirst layer, and wherein the first layer has a lower reflectance thanthe second layer.
 18. The display device according to claim 11, furthercomprising a light diffusion plate and a polarizing plate between theinput device and the first element layer.
 19. The display deviceaccording to claim 11, wherein each of the first transistor and thesecond transistor has a channel formation region comprising a metaloxide.
 20. An electronic device comprising: the display device accordingto claim 11; a first housing; a second housing; and a hinge, wherein thedisplay device is across the first housing and the second housing thatare joined to each other by the hinge, and wherein part of the displaydevice is capable of being bent.
 21. The display device according toclaim 1, further comprising a third substrate between the first elementlayer and the input device.
 22. The display device according to claim 1,wherein the light-emitting element is overlapped with an opening of areflective electrode of the reflective liquid crystal element.
 23. Thedisplay device according to claim 11, further comprising a thirdsubstrate between the first element layer and the input device.
 24. Thedisplay device according to claim 11, wherein the light-emitting elementis overlapped with an opening of a reflective electrode of thereflective liquid crystal element.